I crossed out all the prime numbers

i need to fine the largest prime number

and this guestion
A movie theater runs two documentary films continuously. One documentary runs for 40 minutes and a second documentary runs for 60 minutes. Both movies begin at 3:00 p.m. When will the movies begin again at the same time?

1st question; There is no largest prime number. Whatever you select, there will always be a larger one

2nd question:
2 x 60 = 3 x 40 = 120
(least common multiple)

Let's look at your movie theater question.

The first film starts at 3:00, 3:40, 4:20, 5:00, 5:40, 6:20, 7:00.

What times does the second film start? When will the movies begin at the same time?