Even or odd from the last digit

In your book, Maths Mela (Chapter 3), the numbers from 1 to 100 are set out on crayon strips, in pairs. Look at them in order and you see a pattern: odd, even, odd, even, … Even and odd numbers take turns.

So we do not need to pair up 154 objects to know whether 154 is even. Only the last digit, the ones digit, matters. If the ones digit is 0, 2, 4, 6 or 8, the number is even. If it is 1, 3, 5, 7 or 9, the number is odd. So 30, 46, 78, 154 and 300 are even, and 67, 99 and 415 are odd. The first digit does not decide it: 154 starts with an odd 1 but ends in 4, so it is even.

Shirley notices that the numbers just before and just after an odd number are both even: before 67 comes 66 and after it comes 68, and both are even. Shiv wonders whether the numbers just before and just after an even number are both odd. They are! Before 30 comes 29 and after it comes 31, both odd. That is because even and odd take turns.

Write 10 numbers in order without skipping any, starting from 20: 20, 21, 22, 23, 24, 25, 26, 27, 28, 29. The even ones are 20, 22, 24, 26, 28 and the odd ones are 21, 23, 25, 27, 29: five of each, taking turns.