x * (x + 5) * (x + 10) * (x + 15) = 15000
x^4 + 30 x^3 + 275 x^2 + 750 x = 15000
x^4 + 30 x^3 + 275 x^2 + 750 x - 15000 = 0
The roots are x = 5, x = -20, x = -15/2 + (5/2)(sqrt(15))i, and x = -15/2 - (5/2)(sqrt(15))i
The only natural number (non-negative integer) solution is x = 5, so:
The answer is 5, 10, 15, 20.