If both trains just left the station on parallel tracks, then both leave the station together again;
We will solving for the LCM (Least Common
Multiple),
The LCM of 12 and 9
12=2×3×4x6 × 12
9=3x1 x 9
The LCM of 12 and 9 is 36
Hence, Both trains will leave the station together again in B. 36 minutes.