SOLUTION: Josh left campus and drove toward San Francisco at an average speed of 40km/h. Rob left some time later driving in the same direction at an average speed 50 km/h. After driving for

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Question 847811: Josh left campus and drove toward San Francisco at an average speed of 40km/h. Rob left some time later driving in the same direction at an average speed 50 km/h. After driving for four hours Rob caught up with Josh. How long did Josh drive before Rob caught up?
Answer by josgarithmetic(39621) About Me  (Show Source):
You can put this solution on YOUR website!
Uniform Rates Travel Equation, R%2AT=d, the rates being kilometers per hour.

Let t = "some time later", a quantity of time (not a time point).
Let d = the distance when they traveled when Rob meets Josh.

________________speed___________time___________distance
Josh____________40______________t+4__________d=40%28t%2B4%29
Rob_____________50______________4____________d=50%2A4

Equate the two expressions for d. Solve for t.
Note that they "meet" when Josh traveled for t%2B4 hours.