Odd and even

In your book, Maths Mela (Chapter 4, We the Travellers — II), we sort numbers into even and odd. Put 18 dots in pairs: they make 9 pairs with nothing left over, so 18 is even. Put 23 dots in pairs: they make 11 pairs with 1 dot left over, so 23 is odd.

We can also look at the ones digit. A number is even if its ones digit is 0, 2, 4, 6 or 8, and odd if it is 1, 3, 5, 7 or 9. So 498, 724, 100 and 846 are even, and 297, 199, 789 and 6,893 are odd.

Add 2 to 18 and you get 20: one more pair, and still nothing left over, so it stays even. Add 2 to 23 and you get 25: one more pair, and still 1 left over, so it stays odd.

What about adding two numbers? Even + even is even: 12 + 6 = 18, because pairs added to pairs are still pairs. Odd + odd is even: 13 + 9 = 22, because the two left-over dots join to make a new pair. Odd + even is odd: 7 + 12 = 19, because the one left-over dot has no partner.

Raghu plays with his grandfather's torch. The first press turns it ON, the second turns it OFF, the third turns it ON, and so on. After every odd-numbered press (1st, 3rd, 5th …) it is ON, and after every even-numbered press (2nd, 4th, 6th …) it is OFF. 23 is odd, so after the 23rd press the torch is ON.