Originally Posted by

**Boris B** Okay, I figured it out. You pointed out my antiderivative was correct. In order to get from that to the actual distribution function, we had to correct by a factor: the proportionality constant. I had forgotten to take into account the fact that the integral (0 to 40) f(x) dx = 1, because it's a probability question.

$\displaystyle C \int_0^40 f(x) dx = 1$

$\displaystyle 1 = \int_0^40 C (10+x)^{-2} dx = -C (10+x)^{-1} = C/10 - C/50$

My answer was off by exactly that factor.