wat is the highest # that n can be if 90!/2^n is a whole #
[] stand for floor function.
The equation comes from counting all even numbers in 90! that is $\displaystyle [\frac{90}2]$
Then counting all multiples of 4 that give an additional multiple of 2 in 90!, that is $\displaystyle [\frac{90}4]$
Then counting all multiples of 8 that give yet another additional multiple of 2 in 90!, that is $\displaystyle [\frac{90}8]$
And so on....