Question

Luke now takes 20 cards numbered 1 to 20. Show how the cards can

be paired to give an LCM of 60.

Answer 1

Explanation:

The pair members cannot be multiplied. Look at the last numbers that are prime starting with eleven

What are you going to pair these primes with

11 13 17 19

So they must be added (or maybe subtracted).

1 19

2 18

3 17

4 16

5 15

6 14

7 13

8 12

9 11

and 20 which no matter how you do this, there will be 1 card left over.

Each of them adds to 20 so the LCM is 20