Two trains of the same length take 10 seconds and 15 seconds respectively to run past a signal post. Running on parallel tracks with the faster one overtaking the slower, they take how many seconds to pass each other completely?

Each train's speed is its own length ÷ its pole time, so they are L/10 and L/15 m/s and the overtaking speed is L/10 − L/15 = L/30 m/s. The distance to be covered is both lengths, 2L, so the time is 2L ÷ (L/30) = 60 seconds. The familiar answer 12 seconds belongs to the case where the trains run towards each other, when the speeds add instead of subtracting; 25 is just 10 + 15.