Amy is standing still on the ground; bill is riding his bicycle at 5 m/s eastward; and carlos is driving his car at 15 m/s westward. how fast does bill see carlos moving and in what direction?

20 m/s westward

Explanation:

Taking eastward as positive direction, we have:

is the velocity of Bill with respect to Amy (which is stationary)

is the velocity of Carlos with respect to Amy

Bill is moving 5 m/s eastward compared to Amy at rest, so the velocity of Bill's reference frame is

Therefore, Carlos velocity in Bill's reference frame will be

and the direction will be westward (negative sign).

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