Two trains leave a station at the same time. one train returns to the station in 45 minutes and leaves again. The other train returns in 75 minutes and leaves again. after how many minutes will both trains return to the station at the same time?
You basically just have to find the lcm of 45 and 75, which is 225.
This means after the two trains leave the station, they will both be back at the same time after 225 minutes.