Counting in equal groups

In your book, Maths Mela (Chapter 3), Gundappa has some land with tall coconut trees. Seen from the top, the trees are planted in neat blocks. Each block has 3 rows of 3 trees, which is 9 trees. There are 9 such blocks. How many trees in all?

Counting 81 trees one by one is slow, and it is easy to lose count. Instead, count equal groups. There are 9 blocks with 9 trees in each: 9 × 9 = 81. So Gundappa has 81 coconut trees.

Gundappa plucks 5 coconuts from each tree. That is 81 groups of 5, or 81 × 5. Break 81 into 80 and 1: 80 × 5 = 400 and 1 × 5 = 5. Together, 400 + 5 = 405 coconuts.

Equal groups also help when trays are stacked one on top of another, all with the same arrangement. Count what is on one tray, then multiply by the number of trays. If one tray holds 3 rows of 5 cups, it has 15 cups; 4 such trays hold 15 × 4 = 60 cups.