algebra
posted by sydneeey .
Truck A leaves a truck stop traveling at 80 km/hr. Four hours later, truck BB leaves the same truck stop traveling in the same direction at 90 km/ hr. How long does it take truck B to catch up to truck A?

let the time traveled by truck A be t hours, then A went 80t km
clearly the time traveled by truck B must be t4 hours,
Didn't he then go 90(t4) km
Now I wonder how far each of them went, ...
mmmmhhh? 
The distance they travel is the same, and the time is 4 hours different.
DistanceA=80*time
DistanceB=90*(time4)
set them equal, and solve for time. That is the total time, if you want the time B was traveling, subtract four hours.