Multiplying by tens and hundreds

In your book, Maths Mela (Chapter 9), we count wheels. 10 tricycles with 3 wheels each have 10 × 3 = 30 wheels. 10 more tricycles have 30 more wheels. So 20 tricycles have 30 + 30 = 60 wheels: 20 × 3 = 60.

10 cars with 4 wheels each have 10 × 4 = 40 wheels. 30 cars have 40 + 40 + 40 = 120 wheels. 30 × 4 is three times 10 × 4. When the number of groups is a multiple of 10, use the small fact and make it ten times bigger: 6 × 8 = 48, so 60 × 8 = 480.

It works for hundreds too. 100 bikes with 2 people on each carry 100 × 2 = 200 people. 100 cars with 4 people in each carry 400 people, so 500 cars carry 5 times as many: 2000 people. 500 × 4 = 2000.

Look at the pattern: 3 × 9 = 27, 30 × 9 = 270 and 300 × 9 = 2700. The product becomes ten times bigger, then a hundred times bigger, just like the number of groups.