A Volvo, a Rover and a Porsche are each travelling with constant velocity along a straight road.?

Question

The Volvo is travelling in the same direction as the Rover but 10 miles per hour faster while the Porsche
is travelling in the opposite direction with a speed of 100 miles per hour relative to the Volvo. What
is the velocity of the Porsche relative to the Rover? What is the velocity of the Rover relative to the
Porsche?

Answer 1

I'll write Vab to mean the velocity of a with respect to b
let the volvo/rover direction be positive
and the porsche direction as negative
then you're given
Vvr = 10mi/h
Vpv = -100mi/h
so
Vpr = Vpv + Vvr = -100 + 10 = -90mi/h
that means 90mi/h in the porsche direction
and
Vrp = -Vpr = 90mi/h
that means 90mi/h in the volvo/rover direction

,.,.,.

Answer 2

Let V be the Volvo's velocity, R the Rover's and P the Porche's:
V=R+10
P-V=100 = P-(R+10)
100=P-R-10 So P-R=90
The velocity of the Porsche relative to the
Rover is the same as the velocity of the Rover
relative to the Porsche: 90mph but in the opposite direction

Answer 3

Velocity of Porsche relative to Rover = 90 mph
Velocity of Rover relative to porsche is also 90 mph, but in the opposite direction.