Most Common ISEE Math Mistakes (and How to Fix Them)

Why Math Mistakes Matter More on the ISEE

The two math sections account for 84 of the 160 scored questions on the ISEE — over half the test. Small errors add up quickly, and many students lose more points to avoidable mistakes than to genuinely hard problems.

The good news: most math mistakes fall into predictable patterns. Recognizing these patterns is the first step to eliminating them.

Mistake 1: Misreading the Question

The single most common error. Students solve the problem correctly but answer the wrong question.

Examples:

• The question asks for the perimeter, but the student calculates the area
• The question asks "how many more" but the student gives the total
• The question asks for the value of 2x, but the student gives x

Fix: After solving, reread the question and check that your answer matches what was asked. Circle or underline the key word in the question before you start solving.

Mistake 2: Fraction and Decimal Errors

Fractions appear throughout the ISEE, and small mistakes compound:

• Forgetting to find a common denominator before adding fractions
• Flipping the wrong fraction when dividing (multiply by the reciprocal of the divisor, not the dividend)
• Converting between fractions and decimals incorrectly (1/3 ≠ 0.3, it's 0.333...)
• Forgetting that multiplying two fractions less than 1 gives a smaller result, not larger

Fix: Practice fraction operations daily for two weeks. It's boring but effective. Use math topic practice to drill specific fraction skills.

Mistake 3: Negative Number Errors

Negative numbers trip up students more than almost any other concept:

• Subtracting a negative: 5 − (−3) = 8, not 2
• Multiplying negatives: (−2)(−3) = 6, not −6
• Squaring negatives: (−3)² = 9, but −3² = −9 (the parentheses matter)
• Distributing negatives: −(x − 3) = −x + 3, not −x − 3

Fix: When you see a negative sign, slow down. Write out each step instead of doing it in your head.

Mistake 4: Percentage Confusion

Percentage questions appear on every ISEE, and the errors are predictable:

• Finding 20% of 80 as 80 ÷ 20 = 4 instead of 80 × 0.20 = 16
• Confusing "percent of" with "percent increase" (a 25% increase on 80 is 100, not 20)
• Applying percentage decrease then increase and assuming they cancel out (they don't — 100 decreased by 10% then increased by 10% gives 99, not 100)

Fix: Always convert percentages to decimals before calculating. "25% of X" means 0.25 × X. Practice the distinction between "percent of" and "percent change."

Mistake 5: Geometry Formulas

Students either memorize formulas incorrectly or apply the right formula to the wrong shape:

• Confusing radius and diameter (area = πr², not πd²)
• Using perimeter formula for area or vice versa
• Forgetting to halve the base × height for triangles (A = ½bh, not bh)
• Mixing up surface area and volume formulas for 3D shapes

Fix: Make a formula sheet and quiz yourself. Better yet, understand why each formula works — students who understand the logic rarely mix them up.

Mistake 6: Not Estimating First

Many students dive into calculation without checking if their approach makes sense. Estimating first catches errors early:

• If a question asks for 48% of 200, the answer should be close to half of 200 — about 100. If you get 9.6, something went wrong.
• If two angles in a triangle are 40° and 60°, the third should be 80°. If you get 160°, you added instead of subtracting.
• If a rectangle is 3 by 7, the area should be around 20. If you get 200, check your work.

Fix: Before calculating, take 5 seconds to estimate the approximate answer. After calculating, check if your answer is in the right ballpark.

Mistake 7: Quantitative Comparison Traps

The quantitative comparison questions on the ISEE have specific traps that catch unprepared students:

• Assuming a variable is positive — unless stated, a variable could be negative, zero, or a fraction
• Choosing (D) too quickly — "cannot be determined" is correct only when different valid values give different results
• Choosing (D) too rarely — some students never pick this option, missing questions where it's correct
• Calculating when reasoning would work — if Column A is clearly always greater, you don't need the exact values

Fix: For every comparison question with variables, test at least three values: a positive number, zero or a negative, and a fraction between 0 and 1.

Building Better Habits

The pattern across all these mistakes: rushing causes more errors than difficulty. Students who slow down, read carefully, and check their work consistently outscore students who race through.

Daily practice habits that reduce errors:

1. Do 10 problems with no time pressure, focusing on accuracy. Then redo the same types under time pressure.
2. Keep an error log. Write down every mistake, categorize it, and review the log weekly.
3. Practice mental math. The ISEE doesn't allow calculators, so fluency with basic operations saves time and reduces errors.
4. Use topic-specific practice to target the areas where your child makes the most mistakes.