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?

The 60th customer will 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.