First you have to find the common multiple of 6 and 15 which is 30, then you have to multiply numerator. So for 1/6 you would multiply it by 5, which means 1/6 will turn into 5/30, and for 4/15 you would need to multiply it by 2, which mean 4/15 would change into 8/30. Then, you need to add 8+5 which would give you 13. So that means your final answer should be 13/30.
Without paying for the company shirt, you would make 8.25 * x = $8.25x. Spending $15 can be modeled by the number -15 so including the company shirt, you would make $(8.25x - 15) per week.