Since dividing by a fraction is done by multiplying by the fraction's reciprocal:
x times 50/3 is an integer.
x times 125/1 is an integer.
x times 25/4 is an integer.
If all these multiplications are integers, then the factors of each denominator must be canceled by factors of x. So let's look at the factors of the denominators (of the reciprocals):
3 = 3 * 1
1 = 1 * 1
4 = 2 * 2
Since 1 is a factor of everything, x must only have one 3 and two 2's as factors in order for it to cancel the factors of each of the denominators. (any additional factors will just make x larger.) So:
x = 3 * 2 * 2 = 12