Ok, first is 100 divisible by 50? Yes, so you have to multiply 50 by 2 to get 100, so (2 x 9 = 18) nearest 20 so thats a nono. (21 x 2 = 42) nearest 20, also a nono. (22 x 2 = 44) nearest 40 so yes. (8 x 2 = 16) nearest 15 so also yes.
Hope this helps!
