The red line train arrives every 8 minutes and the blue line train arrives every 10 minutes at the station. when will they next arrive at the station at the same time

In 40 minutes.  Every 8 minutes translates to 8, 16, 24, 32 and 40 minutes later.  Every 10 minutes is 10, 20, 30 and 40 minutes later.  Both arrive at the same time when 40 minutes have passed.

If you need an equation, it would be y=2x+8, y would be how long it grew, x being weeks.if you need how long he was asleep, it would be 4 weeks if he started with no beard, the equation would be x=8÷2if he did have a beard before hand, the equation would be 2w+x=8