Two trains having equal lengths take 10 seconds and 15 seconds respectively to cross a post If the l...........

Explanation:

Speed of train 1 = (120/10)m/sec = 12 m/sec 

Speed of train 2 =(120/15)m/sec = 8 m/sec 

if they travel in opposite direction, relative speed = 12 + 8 = 20 m/sec
distance covered = 120 + 120 = 240 m
time = distance/speed = 240/20 = 12 sec

Hence, option 1 is the correct answer.