I've been stuck on this question for a while...

Joe has a midterm in his 1:00pm geography class. Unfortunately, his computer class, which is scheduled to end at 12:50pm, usually runs overtime. The amount of time it takes that it ends late is normally distributed with a mean of 3 minutes and a standard deviation of 2 minutes. It then takes him an amount of time to get to his geography classroom which is normally distributed with mean 4 minutes and standard deviation 1 minute. What is the probability that Joe is late for his midterm? (Use the fact that the sum of two independent normal random variables is also normally distributed.)

Thanks