Adding with regrouping

In your book, Maths Mela (Chapter 7), Daisy and Lou go by bus on a school trip to Mawlynnong. 24 teachers go from their school and 28 teachers from a neighbourhood school. How many teachers in all? First estimate: 24 is about 20 and 28 is about 30, so about 50.

Now add in columns, ones first. 4 ones + 8 ones = 12 ones. 10 ones make 1 ten, so 12 ones is 1 ten and 2 ones. Write 2 in the ones place and add the 1 ten to the tens. 2 tens + 2 tens + 1 ten = 5 tens. So 24 + 28 = 52. Changing 10 ones into 1 ten is called regrouping. Our estimate, about 50, was close.

Bigger numbers work the same way. 438 children from their school and 476 from the other school go too. Estimate: about 400 + 500 = 900. Ones: 8 + 6 = 14, that is 1 ten and 4 ones. Tens: 3 + 7 + 1 = 11 tens, that is 1 hundred and 1 ten. Hundreds: 4 + 4 + 1 = 9 hundreds. So 438 + 476 = 914.

Sometimes we regroup and get a 0. Daisy and Lou had ₹185 in their piggy bank and their mother gave them ₹125 more. Ones: 5 + 5 = 10, that is 1 ten and 0 ones. Tens: 8 + 2 + 1 = 11 tens, that is 1 hundred and 1 ten. Hundreds: 1 + 1 + 1 = 3. They took ₹310 for the trip.