Car Collision Time Calculation

[drunkenfool]

I understand that this is a REALLY simple question, but for the life of me, I cannot figure out how to get the right answer.

"Two cars, A and B, are 400. meters apart. Car A
travels due east at 30. meters per second on a
collision course with car B, which travels due
west at 20. meters per second. How much time
elapses before the two cars collide?"

Hopefully, some of you helpful souls out there can help me out. I hope no ridicule comes from this. :(

What I have done is find out how much time Car A would take to travel 400 M.
400/30 = 13.333 s

Now I know that they WILL collide atleast before 13.333 s. Other than that, I'm stuck. Thanks in advance.

 

[drunkenfool]

Ah, oh my God. I know why I didn't get the right answer now. I've been putting 20t = 30t. It had never occurred to me that 20t + 30t is equal to 400. Damn, I feel stupid now.

 

[mukundpa]

It happens some time.