this is about as straightforward a probability problem as there could be. If you don't even know where to start you need to pay more attention in class or to your textbook.

Take your value, 60 lb/ft^2, normalize it according to the mean and deviation of your snow load distribution, 50 and 8 lb/ft^2, also known as finding it's z-score, and use the table of the CDF of the standard normal to find your probability.

Note it will be 1 - Pr[snow load < 60 lb/ft^2].