Use the Following Information for Questions

Multiple Choice

Use the following information for questions:
The time it takes a student to finish buying his/her text books, W, is a normal random variable. This variable W is the sum of two other normal variables, X and Y (W = X + Y) , where X = the time to wait in line at the ATM machine to get cash, and Y = the time to wait in line at the cashier to buy the books. Assume that X and Y are independent normal random variables, with the following means and standard deviations:
-What is the standard deviation for the time it takes a student to finish buying his/her text books?

A) 5 minutes
B) 12 minutes
C) 7 minutes
D) 13 minutes