A cyclist riding on a straight, level road sees a car pass her heading east. The cyclist measures the car’s speed as 10 m/s east relative to her. If the car’s speed relative to the ground is 25 m/s east, what is the cyclist’s speed relative to the ground?
a) 35m/s east
b) 10m/s east
c) 15m/s east
d) 15m/s west