Estimation and Rounding

GRE Quantitative · Axiom Academy

LESSON Estimation and Rounding Rounding is a mechanical rule, not a guess — master the digit test and you can turn ugly arithmetic into fast, reliable estimates. 1. The Rule Lives in One Digit Every round comes down to a single question: look at the digit one place to the right of where you're rounding. 5 or more snaps the target digit up; less than 5 leaves it alone and drops everything after it. Watch the deciding digit light up, then watch the number snap to its rounded form. 24.67 — the hundredths digit (7) decides the tenths place 7 ≥ 5, so the tenths digit snaps up: 24.7 Rounding isn't just about cleaning up a decimal — it's also how you box in a value you can't compute exactly. To estimate √87 with no calculator, find the two perfect squares it sits between: 9² = 81 and 10² = 100 . Since 87 sits much closer to 81 than to 100, √87 must sit much closer to 9 than to 10. Watch the bracket close in on the true value. 81 < 87 < 100, so 9 < √87 < 10 87 is 6 past 81 but 13 short of 100 — closer to 9 3. Round Both Factors — But Know the Drift For multiplication, round each factor to a friendly number before multiplying. 24 × 37 rounds to 25 × 40 = 1000 — nowhere near exact, but close enough to know the true product (888) lands in the 800s–900s range. The catch: every factor you round adds drift , and if the answer choices are packed close together, that drift can point at the wrong one.

This is the written version of the interactive lesson above. See the full GRE Quantitative course.