Answer:
Step-by-step explanation:
The answer would be to find the LCM of both 3 and 5, which is 15. The 15th customer would receive both a mug and a t-shirt. Let's call mugs "x" and shirts "o" and line them up. The bold x's and o's are the lucky winners:
x x x x x x x x x x x x x x x
o o o o o o o o o o o o o o o
The last x and the last o are both bold for the first time right there, at the 15th letter of each.
