A movie rental gives a free popcorn to every 15th customer and a free movie rental to every 4th customer. Which customer was the first to win both prizes?

In both cases, start with 0.

Every 15th customer includes #15, #30, #45, #60, etc.

Every 4th customer includes #4, #8, #12, #16, #20, #24, #28, #32, #36, #40, #44, #48, #52, #56, #60

#60 is the first customer to win both prizes.

The 60th customer will win both prizes.