math

Reverse percentages by multiplying by 100/p%, in this case 405 * (100/90) = 450.

Simpy verified:  450 * (90/100) = 405.

Mathmatically if Y = N*(X/100) then N = Y * (100/X), where N is your original number, Y is the number after the percentage is taken, and X is the percentage expressed as a number (E.G., 90 for 90%).

By the way, I assume what you meant to ask was "What number is 405 90% of".  If that isn't what you meant, then disregard my answer.  The literal answer to your question is "405 is 450% of 90."

To elaborate, 405 / 90 = 4.5, therefore 405 is 450% of 90.  Any number divided by itself is 100%, E.G., 90 is 100% of 90.

Sorry for multiple answers, but I don't see any way to edit a response.