Checking calculations: estimate and inverse
Estimate the answer before you calculate, work in a column, check with the inverse operation, and decide whether a result (a calculator's too) is reasonable.
- Germany: grade 4
- Russia: grade 3
Guide
Checking a calculation has three tools: an estimate (round the numbers and work out a rough answer first: 398 + 205 ≈ 400 + 200 = 600), the inverse operation and the calculator. In Russian schools these are taught in Grades 3 and 4.
After a column calculation, compare the result with the estimate. If it is far off, look for a mistake. Check addition by subtracting, subtraction by adding (difference plus the subtracted number gives the starting number), multiplication by dividing and division by multiplying.
A calculator can give a wrong result too, if an extra digit or the wrong key is pressed. So compare its result with the estimate. A calculator helps to check, but it does not replace being able to calculate.