word problem

[86FX]

A car and train leave at the same time to travel to a destination 390 miles away.
If the speed of the car averages twice the speed of the train and the car arrives 6.5 hours before the train, find the speed of the car and the speed of the train.

Do I start with 390/r^2 + 6.5 = 390/r ??

 

[Mrspi]

A car and train leave at the same time to travel to a destination 390 miles away.
If the speed of the car averages twice the speed of the train and the car arrives 6.5 hours before the train, find the speed of the car and the speed of the train.

Do I start with 390/r^2 + 6.5 = 390/r ??

It appears that you have let r = speed of train.
The speed of the car is twice the speed of the train, so the car's speed would be 2r, NOT r^2.

(390 / 2r) + 6.5 = 390/r

 

[Denis]

When the car arrived, the train had travelled half the distance, or 195 miles.

So train travelled last 195 miles in 6.5 hours: so train speed = 195 / 6.5 = 30 mph.