Rounding numbers to the nearest ten or hundred helps with estimation. Before calculating 47 + 38, rounding to 50 + 40 = 90 provides a quick estimate to check if the exact answer is reasonable.
You have practiced rounding numbers to the nearest ten and nearest hundred — finding the friendly, round number closest to an awkward one. Now you will put that skill to work in a real context: estimation. Instead of computing an exact answer and then wondering if it's correct, estimation lets you quickly predict roughly what the answer should be before you calculate, giving you a built-in accuracy check.
Here is the basic move: before solving 47 + 38, round each number to the nearest ten. 47 rounds to 50; 38 rounds to 40. Now the problem becomes 50 + 40 = 90, which you can compute instantly in your head. Your estimate is 90. When you calculate the exact answer, 47 + 38 = 85, you can ask: is 85 close to 90? Yes — both are in the eighties, so the answer is reasonable. If you had accidentally gotten 185, the estimate of 90 would immediately flag that something had gone wrong.
Estimation is also the right tool when an exact answer isn't needed at all. Suppose you're at a store with $20 and your items cost $3.47, $5.89, and $4.12. You don't need the precise sum — you just need to know if you have enough. Round to $3 + $6 + $4 = $13. Safely under $20, so you're fine. The speed of estimation is the entire point: you trade a small amount of precision for a large gain in convenience.
The choice of which place to round to matters. Rounding to hundreds gives rougher estimates (faster math, more error); rounding to tens gives closer estimates (slightly slower, less error). For 247 + 389, rounding to hundreds gives 200 + 400 = 600, while rounding to tens gives 250 + 390 = 640. The exact answer is 636, so the finer rounding is closer. Choosing the right level of precision for the situation is a judgment that improves with practice — and it is one of the most useful mathematical habits you can develop.