Rounding & estimating

At the checkout you want to know if your money will cover it before the total appears on the screen. While cooking you estimate whether there is still time for dessert. That is exactly what rounding is for: you turn awkward numbers into round ones and can then calculate in your head, fast and good enough.

Rounding to tens

When rounding to tens you ask: which ten is closest? 43 is closer to 40, so it becomes 40. 47 is closer to 50, so it becomes 50. The rule is simple: look at the ones digit. If it is less than 5, round down, so 84 becomes 80. If it is 5 or more, round up. When the digit is exactly 5 the number sits right in the middle and neither ten is closer. For that case the agreed convention is to round up: 85 becomes 90.

Rounding to hundreds

The same game works one level up. When rounding to hundreds you look at the tens digit: 312 has a 1 there, so you round down to 300. In 680 there is an 8, so it goes up to 700. This works at any size: 8472 has a 7 in the tens place, so rounded to hundreds it is 8500.

Estimating: guess first, then calculate

An estimate is a calculation with rounded numbers. Instead of 398 + 205 you calculate 400 + 200 = 600 and know right away: the result is around 600, and the exact value is 603. The same trick works for multiplication: 29 × 4 becomes 30 × 4, and since 3 × 4 = 12 you simply attach the zero and land at 120. An estimate is also your best protection against typos: if the calculator shows 6030 but your estimate says 600, a key slipped somewhere.