AI Tutor
Imagine buying 28 packs of stickers that cost 31each.Youfinishyourlongmathonpaperandget∗∗31 each. You finish your long math on paper and get **8,680** — but did you add an extra zero by mistake?
Before doing big calculations, smart mathematicians use a clever shortcut called an estimate, which means finding a quick, close guess.

Rounding to Friendly Numbers

To estimate fast, we round each number to the nearest ten to make numbers ending in zero that are super easy to multiply in your head.
If the ones digit is 5 or more, we round up to the next ten. If it is 4 or less, we round down.
So 28 becomes 30, and 31 becomes 30. How do we multiply those friendly numbers in a flash?

The Zero Trick for Quick Products

To solve 30×3030 \times 30, multiply the front digits (3×3=93 \times 3 = 9), then attach the two zeros at the end to get 900.
Now look back at that sticker total: is $8,680 close to 900? Not at all! The estimate instantly warned us that an extra zero crept in.
What does checking an answer look like when we solve a full problem from start to finish?

Worked Example: Check the Reasonableness