Two trains having equal lengths, take 10 seconds and 15 seconds respectively to cross a post. If the length of each train is 120 meters, in what time (in seconds) will they cross each other when traveling in opposite direction?

Speed of train 1 =  $\frac{120}{10}m/sec$ = 12 m/sec 

Speed of train 2 = $\frac{120}{15}m/sec$  = 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