Take the above question. In the solution it says there 18 factors of 2 in the number. But how do they work this out? I see no reasoning or intuition between the line of working 10 + 5 + 2 + 1 = 18. Could someone please explain how they came to this conclusion in depth?
They come to that because 10 numbers between 0 and 21 are even, half of those are divisible by 4. Just under half of those will be divisible by 8, and so on.