The first nine multiples of a number [tex] x [/tex] are:
[tex] x, 2x, 3x, 4x, 5x, 6x,7x,8x,9x [/tex]
If we want all of these numbers to be less than 40, we must ask
[tex] 9x < 40 \iff x < \cfrac{40}{9} = 4.\overline{4} [/tex]
This means that every number less than 4 will do the trick:
If you choose 2, you have the following multiples:
[tex] 2,4,6,8,10,12,14,16,18 [/tex]
If you choose 3, you have the following multiples:
[tex] 3,6,9,12,15,18,21,24,27 [/tex]
If you choose 4, you have the following multiples:
[tex] 4,8,12,16,20,24,28,32,36 [/tex]
