6.1 Fraction Arithmetic, Common Denominators & Mixed Numbers

6.1 Fraction Arithmetic, Common Denominators & Mixed Numbers

Fraction arithmetic represents one of the most heavily tested domains on the Wonderlic Basic Skills Test Quantitative section (WBST-QS). Candidates must solve computational and applied fraction problems in an environment that strictly prohibits calculators, operating under an average time allowance of just 26.67 seconds per question.

Achieving speed and absolute computational accuracy requires moving beyond trial-and-error arithmetic. You must master the foundational structure of rational numbers, automate the identification of Least Common Denominators (LCD), execute flawless regrouping during mixed-number subtraction, and leverage aggressive cross-canceling before performing multiplication.

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Taxonomy of Fractions: Anatomy and Core Classifications

A fraction represents a numerical part of a whole or a ratio between two integers, expressed in the standard form $\frac{a}{b}$:

• Numerator ($a$): The upper integer, indicating how many equal parts or portions are being considered.
• Denominator ($b$): The lower integer ($b \neq 0$), indicating the total number of equal parts into which the whole unit is divided.
• Fraction Bar (Vinculum): Functions mathematically as a division symbol ($a \div b$).

On the WBST, test items utilize three distinct fraction representations:

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Converting Between Mixed Numbers and Improper Fractions

Direct computation with mixed numbers during multiplication and division is impossible without first transforming them into improper fractions. Conversely, final question options on the WBST are almost universally presented as simplified mixed numbers.

1. Mixed Number to Improper Fraction Conversion

To convert a mixed number $W\frac{n}{d}$ into an improper fraction, multiply the whole integer $W$ by the denominator $d$, add the numerator $n$, and place the resulting sum over the original denominator $d$:

$\text{Improper Fraction} = \frac{(W \times d) + n}{d}$

• Example A: Convert $4\frac{3}{5}$ into an improper fraction. $\frac{(4 \times 5) + 3}{5} = \frac{20 + 3}{5} = \frac{23}{5}$
• Example B: Convert $7\frac{5}{8}$ into an improper fraction. $\frac{(7 \times 8) + 5}{8} = \frac{56 + 5}{8} = \frac{61}{8}$

2. Improper Fraction to Mixed Number Conversion

To convert an improper fraction $\frac{a}{b}$ into a mixed number, divide the numerator $a$ by the denominator $b$ using mental integer division. The quotient becomes the whole number, the integer remainder becomes the new numerator, and the denominator remains unchanged:

• Example A: Convert $\frac{37}{6}$ into a mixed number. $37 \div 6 = 6 \text{ with a remainder of } 1 \implies 6\frac{1}{6}$
• Example B: Convert $\frac{59}{8}$ into a mixed number. $59 \div 8 = 7 \text{ with a remainder of } 3 \implies 7\frac{3}{8}$

Pacing Tip: Do not write out long-division brackets on scratch paper for these conversions. Master mental multiplication tables up to $12 \times 12$ so you can identify the quotient and remainder within 3 seconds.

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Finding the Least Common Denominator (LCD)

Fractions cannot be added or subtracted unless they share an identical common denominator, which represents the Least Common Multiple (LCM) of the denominators.

While traditional textbook pedagogy teaches prime factorization factor trees, that method is too slow for the WBST's 26.7-second clock. Instead, use the Largest Denominator Multiple Method:

1. Identify the largest denominator among the fractions.
2. Check whether that number is evenly divisible by all other denominators. If it is, that number is the LCD.
3. If not, mentally recite the consecutive multiples of the largest denominator ($2\times, 3\times, 4\times$) until you reach a multiple that is evenly divisible by the other denominators.

Benchmark Denominator Pairings on the WBST

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Addition and Subtraction with Unlike Denominators

Once the LCD is determined, convert each fraction into an equivalent fraction by multiplying both the numerator and denominator by the factor required to produce the LCD. Add or subtract the numerators while keeping the common denominator unchanged.

Step-by-Step Addition Demonstration

Evaluate: $\frac{5}{6} + \frac{3}{8}$

1. Find the LCD: Multiples of 8: 8, 16, 24. Since 24 is divisible by 6, the LCD is 24.
2. Scale fractions: $\frac{5 \times 4}{6 \times 4} = \frac{20}{24}, \quad \frac{3 \times 3}{8 \times 3} = \frac{9}{24}$
3. Add numerators: $\frac{20}{24} + \frac{9}{24} = \frac{29}{24}$
4. Convert to mixed number: $\frac{29}{24} = 1\frac{5}{24}$

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Borrowing and Regrouping in Mixed Number Subtraction

Mixed number subtraction frequently presents a scenario where the fractional part of the first number (minuend) is smaller than the fractional part of the second number (subtrahend). In such cases, you must borrow 1 from the whole number and convert it into fractional units of $\frac{d}{d}$.

                         THE MIXED NUMBER REGROUPING FORMULA
                         
                                     W n/d
                                       |
                   (Borrow 1 from W)   |   (Convert 1 into d/d)
                                       v
                                (W - 1) + (d/d + n/d)
                                       =
                                 (W - 1) (n + d)/d

Case 1: Subtracting Mixed Numbers with Identical Denominators

Evaluate: $5\frac{1}{4} - 2\frac{3}{4}$

1. Observe fractional parts: $\frac{1}{4} < \frac{3}{4}$. Direct subtraction would produce a negative fraction.
2. Borrow 1 from the whole number 5: $5\frac{1}{4} = 4 + 1 + \frac{1}{4} = 4 + \frac{4}{4} + \frac{1}{4} = 4\frac{5}{4}$
3. Execute vertical subtraction:

  4 5/4
- 2 3/4
-------
  2 2/4 = 2 1/2

Case 2: Subtracting a Mixed Number from a Pure Whole Integer

Evaluate: $8 - 3\frac{5}{12}$

1. Regroup 1 from the whole number 8: $8 = 7 + 1 = 7\frac{12}{12}$
2. Subtract whole numbers and numerators independently: $7\frac{12}{12} - 3\frac{5}{12} = (7 - 3) + \frac{12 - 5}{12} = 4\frac{7}{12}$

Case 3: Subtraction Involving Both Unlike Denominators and Regrouping

Evaluate: $7\frac{1}{6} - 3\frac{5}{8}$

1. Convert to common denominator (LCD = 24): $7\frac{1}{6} = 7\frac{4}{24}, \quad 3\frac{5}{8} = 3\frac{15}{24}$
2. Regroup because $\frac{4}{24} < \frac{15}{24}$: $7\frac{4}{24} = 6 + \frac{24}{24} + \frac{4}{24} = 6\frac{28}{24}$
3. Subtract whole numbers and numerators: $(6 - 3) + \frac{28 - 15}{24} = 3\frac{13}{24}$

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Fraction Multiplication: Pre-Simplifying via Cross-Canceling

When multiplying fractions, never multiply large numerators and denominators straight across before reducing; doing so generates unwieldy figures that require slow long division to simplify. Instead, leverage diagonal and vertical cross-canceling.

The Rules of Fraction Multiplication

1. Convert all mixed numbers into improper fractions. You cannot multiply whole numbers and fractions separately.
2. Cross-cancel common factors: Divide any numerator and any denominator by their greatest common divisor.
3. Multiply remaining numerators and denominators straight across.

Worked Example: Multiplication with Cross-Canceling

Evaluate: $4\frac{2}{3} \times 2\frac{1}{7}$

1. Convert to improper fractions: $4\frac{2}{3} = \frac{14}{3}, \quad 2\frac{1}{7} = \frac{15}{7}$
2. Set up the product: $\frac{14}{3} \times \frac{15}{7}$
3. Cross-cancel diagonally:
   • 14 (numerator) and 7 (denominator) divide by 7: leaving 2 in the numerator and 1 in the denominator.
   • 15 (numerator) and 3 (denominator) divide by 3: leaving 5 in the numerator and 1 in the denominator.
4. Multiply remaining factors: $\frac{2 \times 5}{1 \times 1} = \frac{10}{1} = 10$

By cross-canceling, the entire problem is resolved mentally in under 10 seconds without calculating $14 \times 15 = 210$ and $210 \div 21 = 10$.

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Fraction Division: The Reciprocal Rule ("Keep, Change, Flip")

Dividing by a fraction is mathematically identical to multiplying by its multiplicative inverse (reciprocal). The reciprocal of $\frac{a}{b}$ is $\frac{b}{a}$.

                          THE KEEP, CHANGE, FLIP (KCF) ALGORITHM
                          
           [ KEEP ]                     [ CHANGE ]                     [ FLIP ]
     Leave the dividend          Change division sign (÷)        Invert the divisor
     exactly as it is.           to multiplication (×).          to its reciprocal.
          a / b                           ×                           d / c

Step-by-Step Division Demonstration

Evaluate: $18\frac{3}{4} \div \frac{5}{8}$

1. Convert mixed number to improper fraction: $18\frac{3}{4} = \frac{(18 \times 4) + 3}{4} = \frac{72 + 3}{4} = \frac{75}{4}$
2. Apply KCF (Keep, Change, Flip): $\frac{75}{4} \div \frac{5}{8} = \frac{75}{4} \times \frac{8}{5}$
3. Cross-cancel common factors:
   • Divide 75 and 5 by 5: leaving 15 and 1.
   • Divide 8 and 4 by 4: leaving 2 and 1.
4. Multiply: $\frac{15 \times 2}{1 \times 1} = 30$

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Applied Workplace Fraction Problems

On the WBST, fraction arithmetic is commonly embedded within practical occupational contexts, including construction measurements, liquid pharmacology dosage, and commercial warehouse inventory.

Construction & Fabrication Scenario

Problem Statement: A maintenance technician needs to cut 6 support brackets from an aluminum rail measuring $58\frac{1}{2}$ inches in length. Each bracket requires $8\frac{3}{4}$ inches of material, and each saw cut wastes $\frac{1}{8}$ inch of metal due to the blade width (kerf). After cutting all 6 brackets, how much scrap material remains?

1. Calculate metal used per cut and bracket: $\text{Material consumed per bracket} = 8\frac{3}{4} + \frac{1}{8} = 8\frac{6}{8} + \frac{1}{8} = 8\frac{7}{8} \text{ inches}$
2. Multiply by 6 brackets: $6 \times 8\frac{7}{8} = 6 \times \frac{71}{8} = \frac{3 \times 71}{4} = \frac{213}{4} = 53\frac{1}{4} \text{ inches}$ (Note: On the final cut, some questions specify kerf count; assuming 6 full blade passes equals $53\frac{1}{4}$ in).
3. Subtract consumed material from total length: $58\frac{1}{2} - 53\frac{1}{4} = 58\frac{2}{4} - 53\frac{1}{4} = 5\frac{1}{4} \text{ inches of scrap}$

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High-Frequency Traps on the WBST

• The Denominator Addition Trap: Adding denominators straight across (e.g., $\frac{1}{2} + \frac{1}{3} = \frac{2}{5}$). This error is always featured as a prominent distractor option.
• The Unconverted Mixed Number Trap: Multiplying whole numbers and fractions separately (e.g., calculating $3\frac{1}{2} \times 2\frac{1}{2}$ as $3 \times 2 + \frac{1}{2} \times \frac{1}{2} = 6\frac{1}{4}$). The correct value is $\frac{7}{2} \times \frac{5}{2} = \frac{35}{4} = 8\frac{3}{4}$.
• The Incomplete Borrowing Trap: When borrowing 1 from a whole number, forgetting to add the existing numerator (e.g., borrowing 1 for $5\frac{1}{4}$ and writing $4\frac{4}{4}$ instead of $4\frac{5}{4}$).