John has two jobs. For daytime work at a jewelry store he is paid $200 per month, plus a commission. His monthly commission is normally distributed with mean $600 and standard deviation $40. At night he works as a waiter, for which his monthly income is normally distributed with mean $100 and standard deviation $30. John's income levels from these two sources are independent of each other.
Referring to Table 6-4, for a given month, what is the probability that John's total income from these two jobs is less than $825?
The answer is .0668
The toal mean is 900 and total Std is 50
My question is how did they come up with the total mean and std?