# Thread: A problem on train

1. ## A problem on train

Help needed to solve math homework problem

Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time will they cross each other travelling in opposite direction?

Thanks

2. ## Re: A problem on train

Originally Posted by burgess
Help needed to solve math homework problem

Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time will they cross each other travelling in opposite direction?

Thanks
$v_1=\dfrac {120}{10}=12 m/s$

$v_2=\dfrac{120}{15}=8 m/s$

I'm not sure exactly what "cross each other" means.

Assuming you mean the time from when their fronts are at the same position to the time when their rears are at the same position then effectively what you have is 1 train moving at the sum of the two speeds covering a distance of 240 metres.

$t=\dfrac {240}{12+8}=\dfrac{240}{20}=12s$

3. ## Re: A problem on train

Originally Posted by romsek
$v_1=\dfrac {120}{10}=12 m/s$

$v_2=\dfrac{120}{15}=8 m/s$

I'm not sure exactly what "cross each other" means.

Assuming you mean the time from when their fronts are at the same position to the time when their rears are at the same position then effectively what you have is 1 train moving at the sum of the two speeds covering a distance of 240 metres.

$t=\dfrac {240}{12+8}=\dfrac{240}{20}=12s$