Explanation:
You need to find the least common multiple (LCM) of 30 and 50. First, write the prime factorization of each:
30 = 2×3×5
50 = 2×5²
The LCM must contain all the factors of both, so:
LCM = 2×3×5²
LCM = 150
It will take 150 minutes (or 2 hours and 30 minutes) before two trains depart at the same time again.