When adding the prices of several items, being able to estimate "about 30,000 won" helps you spot calculation mistakes even without knowing the exact total. Choose two numbers, an operation, and a rounding unit, then compare the exact and estimated results.
What is estimation? Before calculating exactly, round numbers to values that are easy to work with and estimate the result. For example, you can predict that 297 + 512 is about 300 + 500 = 800.
Estimation is not for getting the exact answer; it is for checking whether an actual calculation falls within a sensible range. If a calculator or written result is very different from the estimate, that is a sign that something may have gone wrong.
Change the rounding unit below (tens or hundreds) and see how close the estimate is to the actual value.