Grade 2 · Unit 1: Number and place value

Estimating and checking

Is my answer sensible?

How many people?

How many people are at the fair? We cannot count them all, so we estimate. Count one group, then see how many groups there are.

Estimating big numbers

Count a small part exactly: 50 people. The crowd looks about 4 times as big. So there are about 200 people.

Pairs to 100

Tens: 60 + 40 = 100, so 65 needs 35. Learn pairs to 100: 70 and 30, 25 and 75, 48 and 52. Pairs of hundreds make 1000: 400 and 600.

Group in any order

When adding three numbers, we can add any two first. 8 + 3 = 11 is easy, then 17 + 11 = 28. Or make 20 first: 17 + 3 = 20, then 20 + 8 = 28.

Check: is it reasonable?

Before you calculate, estimate. After you calculate, check: is it close to the estimate? Choose the quickest way: in your head, with a drawing or written.

What I learnt

• I can estimate large quantities.
• I know pairs that make 100.
• I can check if an answer is reasonable.