Algebra Question: Finding Distance When a Train and Car Meet

[mikefitz]

Train leaves station going 50MPH
Car leaves station going 70MPH - 2 hours later

How far til car catches train?

This should be so simple, here is what I've done:

d=50t
d=70(t-2)

50t=70(t-2)
50t=70t-140
140=20t

I get t=7 miles for my final answer, but according to the book this is wrong. Why?

 

[dmoravec]

try it this way: when the car starts the train is 100 miles away. the car makes up ground at 20 miles/hour (70-50)... how many hours does it take to catch the train... how many miles did the car go in that time.

 

[daveb]

How did you get miles from that? If
d=50mph*t hours
d=70mph*(t-2)hours

50mph*t hours=70mph*(t-2)hours
50mph*t hours=70mph*t hours-140 miles
140 miles=20mph*t hours
140miles/20mph = t hours
7 hours = t hours

 

[Kurdt]

I think the problem is that you've solved for t when the question asks you to solve for d, the distance.

Of course you can now just put t back into one of the equations.