Ap statistics question?

Question

Can some one please give me instructions and the answer to this problem.

In a manufacturing process there are three stages. The average runtime for stage one is 5.6 minutes with a standard deviation of 1.2 minutes. The mean run time stage two is 12.3 min with a standard deviation of 3.8 min. and, for stage three the mean is 6.4 minutes with a standard deviation of 2.8 min. What is the mean and standard deviation of the entire process?

Iv been stuck on this for a while. Please send me a instructions on how to solve the problem as well as the ans, so i can check.

Answer 1

TOTAL PROCESS MEAN = [MEAN STAGE 1] + [MEAN STAGE 2] + [MEAN STAGE 3]     (5.6 + 12.3 + 6.4 = 24.3)

TOTAL PROCESS STANDARD DEVIATION = SQRT[ (STD DEV STAGE 1)^2 + (STD DEV STAGE 2)^2 + (STD DEV STAGE 3)^3]  (SQRT( (1.2^2) + (3.8^2) + (2.8^2) ) = 4.87)