🔍 Estimate First, Then Calculate

Hello! Before you do a calculation, you can think about roughly what the answer will be. That is called an estimate. Today we will estimate first, then calculate.

Mina’s basket has a book that costs $48 and a notebook that costs $31. First, estimate: 48 is about 50, and 31 is about 30. 50 + 30 = 80. Now the real sum: 48 + 31 = 79. The estimate was very close!

Imagine a family drives 387 km on the first day and 412 km on the second. This time, let’s round to the nearest hundred: 387 is about 400, and 412 is about 400. Estimate: 400 + 400 = 800. The real total is 799.

Estimating helps with subtraction too. 603 is about 600, and 198 is about 200. 600 − 200 = 400. The real answer is 405, very close to the estimate.

Sam worked out 48 + 31 on a calculator, and the screen says 179. Our estimate was 80. 179 is much too far from 80, so there must be a mistake. Checking again, the right answer is 79. An estimate catches mistakes!

When you add in your head, split the numbers into tens and ones. 46 + 23: the tens are 40 + 20 = 60. The ones are 6 + 3 = 9. Then 60 + 9 = 69.

398 is very close to a round number. Let’s give it 2: 398 becomes 400. Now take that 2 away from 125: 125 becomes 123. The total stays the same: 400 + 123 = 523.

19 is close to a round number: 20. Let’s take away 20 first: 52 − 20 = 32. But we took away 1 too many! Let’s give that 1 back: 32 + 1 = 33. So 52 − 19 = 33.

Summary: first round the numbers to estimate, then find the real answer and compare the two. In your head, split numbers up, make a round number, and for subtraction round and adjust. Now try it in Mental Maths!