Tables from 11 to 20

In your book, Maths Mela (Chapter 13, The Transport Museum), pebbles are laid out in 5 rows, with 15 pebbles in each row. How many pebbles are there? This is 5 × 15. An easy way is to split each row into 10 pebbles and 5 pebbles. Then 5 × 15 = 5 × 10 and 5 × 5 = 50 + 25 = 75.

We can build the whole times-15 table this way: 15, 30, 45, 60, 75, 90, 105, 120, 135, 150. Each answer is a times-10 answer plus a times-5 answer. For example, 3 × 15 = 30 + 15 = 45. That is why 45 - 15 = 30, which is 3 × 10: take away the times-5 part, and the times-10 part is left.

There is another way. Tara counts wheels set out in 3 rows of 14. She splits them into two equal groups, each 3 rows of 7. So 3 × 14 = 3 × 7 and 3 × 7 = 21 + 21 = double of 21 = 42.

In the same way, 6 × 14 is the double of 6 × 7. 6 × 7 = 42, so 6 × 14 = 42 + 42 = 84. You can build the tables from 11 to 20 by splitting the groups like this.