Question

100% of what number is 61

Answer 1

Percent means per hundred
100 % = 1
So the answer is 1 x 61 which equals 61

answer = 61

Answer 2

" 100% " means "the whole thing",
and "is" can be replaced by "is equal to".

You have said "The whole thing of a number is equal to 61."
or
                       "61 is equal to the whole thing of a number."

The only number that 61 can be equal to is 61.