A number rolls down the hill to the nearest ten. Rounding first makes sums quick to estimate.
Which end of the hill will it roll to? Tap it.
A number rolls down to the nearer ten.
Halfway, like 35, rolls up to 40.
To the nearest 100, look at the tens: 349 is 300, 350 is 400.
To estimate, round first, then add.
Find the two tens the number sits between, like 40 and 50 for 47. Then pick the one it is nearer to. 47 is 3 away from 50 and 7 away from 40, so it rounds to 50. Looking at the ones digit is a quick way: 5 or more rounds up, 4 or less rounds down.
A number ending in 5 is exactly halfway, so it is just as near to both tens. Everyone agrees to round it up so that we all get the same answer. 35 rounds to 40, and 250 rounds to 300 when rounding to the nearest 100.
Estimating is finding an answer that is close enough, quickly. Round the numbers first, then work it out: 48 + 31 is about 50 + 30, which is 80. It is also a good way to check an exact answer: if you worked out 48 + 31 = 179, the estimate tells you something went wrong.
Children usually start rounding to the nearest 10 at about 7 to 8, move on to the nearest 100 and 1000 at 8 to 9, and use rounding to estimate and check their answers from then on.