In multiplication, we can apply the commutative property, this means that:
abc = acb
Now, we know that:
8 = 4 * 2
This means that:
9*8 = 9 * 4 * 2
This means that
9*8 is twice as much as 9*4 is 36, we can simply multiply this value by 2 to get 9*8
Therefore:
9 * 8 = 2 (9*4) = 2(36) = 72
Hope this helps :)