Subject: mathqa
Question: two bullet trains of equal lengths take 10 seconds and 30 seconds respectively to cross a telegraph post . if the length of each bullet train be 120 metres , in what time ( in seconds ) will they cross each other travelling in opposite direction ?
Option: 
A: 15 sec .
B: 13 sec .
C: 14 sec .
D: 12 sec .
Answer: A