Assuming x + n = 100, x% of n or n% of x equals 25 minus the square of (distance from 50 of x or n) divided by 100

Tl;dr:

N% of X = |x - 50|^2 / 100

So:

N = 49, X = 51

49% of 51 = 25 - |51 - 50|^2 / 100 = 25 - 0.01 = 24.99