I know its a long question. I dont need help with A or D.

I know it’s a long question. I don’t need help with A or D. I just don’t understand the process of finding the other answers so any explanation would help. The mean time it takes to walk to the bus stop is 8 minutes (with a standard deviation of 2 minutes) and the mean time it takes for the bus to get to school is 20 minutes (with a standard deviation of 4 minutes). The distributions are normal.  b. What is the standard deviation of the trip to school?  c. What is the probability that it will take longer than 30 minutes to get to school?  Due to a miscalculation, you realize it actually takes an average of 10 minutes to walk to the bus stop. e. What is the standard deviation of the trip to school?  f. What is the probability that it will take longer than 30 minutes to get to school?