Easy ways to add and subtract

Your book, Maths Mela (Chapter 10), asks us to find easy ways to add and subtract, without writing the sum in columns. Take 8787 - 99. 99 is just 1 less than 100. So subtract 100, then add 1 back: 8787 - 100 = 8687, and 8687 + 1 = 8688.

Look for numbers that fit together. In 4596 + 104, the 96 and the 4 make 100, so 4596 + 104 = 4700. In 3459 + 21, the 9 and the 1 make 10: 3459 + 21 = 3480. And 260 + 240 = 500.

We can compare without calculating. 54 + 97 and 54 + 90 both start with 54, but 97 is more than 90, so 54 + 97 > 54 + 90. In 84 - 68 and 90 - 68, the same number is taken away, but 84 is less than 90, so 84 - 68 < 90 - 68. In 73 - 54 and 73 - 56, we start from the same 73; taking away less leaves more, so 73 - 54 > 73 - 56.

A fact we know can give us a new one. 139 + 175 = 314, so 314 - 175 = 139. 845 - 394 = 451, so 845 - 395 is 1 less: 450. And 354 + 167 = 521, so 354 + 168 is 1 more: 522.