SOLUTION: A car traveling at 80 kilometers per hour is passed by a second car going in the same direction at a constant speed. After 30 seconds, the two cars are 500 meters apart. Find the s

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Question 894874: A car traveling at 80 kilometers per hour is passed by a second car going in the same direction at a constant speed. After 30 seconds, the two cars are 500 meters apart. Find the speed of the second car.
Found 2 solutions by ankor@dixie-net.com, richwmiller:
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
A car traveling at 80 kilometers per hour is passed by a second car going in the same direction at a constant speed.
After 30 seconds, the two cars are 500 meters apart.
Find the speed of the second car.
:
let s = speed of the 2nd car in meters/sec
:
Change 80 km/hr to meters/sec: %281000%2A80%29%2F3600 = 22.22 meters/sec
then
(22.22-s) = the relative speed between the two cars
:
Write a distance equation, dist = time * speed
30(22.22-s) = 500
666.67 - 30s = 500
-30s = 500 - 666.67
-30s = -166.67
s = -166.67/-30
s = +5.56 meters/sec, speed of the second car
Change to km/h
%283600%2A5.56%29%2F1000 = 20 km/hr

Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!

change 500 meters to kilometers
500 meters is 1/2 k
(80+r)*(30/3600)-(80*30/3600)=1/2
30r/3600=1/2
r/120=1/2
r=60
so the second car was going 80+60=140 kph
check
(140)*(30/3600)-(80*30/3600)=1/2
7/6-4/6=3/6
3/6=1/2 km
ok


The other tutor didn't read the question correctly.
The key is that the problem states that the second car passes the first car.
A car going 20 kph will NOT pass a car going 80 kph