Originally Posted by

**Kasper** I've got this problem here, but I'm a little concerned about my answer, not sure where I'm going wrong.

Let $\displaystyle X_1$ be the time throwing the wheels.

$\displaystyle \mu_X_1 = 40 min$

$\displaystyle \sigma_X_1 = 2 min$

Let $\displaystyle X_2$ be the time firing.

$\displaystyle \mu_X_2 = 60 min$

$\displaystyle \sigma_X_2 = 3 min$

$\displaystyle Y=X_1 + X_2$

$\displaystyle E(Y)=E(X_1)+E(X_2)=40min + 60min = 100min$

$\displaystyle V(Y)=\sigma_X_1 ^2 + \sigma_X_2 ^2 = 2^2 + 3^2 = 13min^2$

$\displaystyle \therfore P(Y<=85)=P(z< \frac{85-100}{sqrt(13)})=0.0000159$

This answer seems extremely low, considering 85 is just outside the first standard deviation. Not sure where I messed up here.