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.