How the way we’re taught to round numbers in school falls short

Imagine needing to estimate the total cost of the items in your grocery basket to decide whether to put something back. So you round to the nearest dollar for each potential purchase, using the “round-to-nearest” technique commonly taught in school. That prompts you to round each item’s cost up if the change portion is at least 50 cents and round down if less.

This rounding approach works well for quickly estimating a total without a calculator. And it yields the same results when a particular rounding task is repeated. For instance, rounding 4.9 to the nearest whole…