Each morning a man goes for a walk. He walks to the top of a hill and then walks down again. The time he takes to walk up the hill is normally distributed with µ = 20 (minutes) and σ = 3, whereas the...

Each morning a man goes for a walk. He walks to the top of a hill and then walks down again. The time he takes to walk up the hill is normally distributed with µ = 20 (minutes)  and σ = 3, whereas the time he takes to walk down the hill is normally distributed with µ = 15 and σ = 4. Assuming that the time taken for the uphill and downhill walks are  independent, what is the probability that his walk will last:

a) between 35 and 40 minutes?

b) less than half an hour?