Answer:
20 m/s westward
Explanation:
Taking eastward as positive direction, we have:
[tex]v_B = +5 m/s[/tex] is the velocity of Bill with respect to Amy (which is stationary)
[tex]v_C = -15 m/s[/tex] 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
[tex]v_B = +5 m/s[/tex]
Therefore, Carlos velocity in Bill's reference frame will be
[tex]v_C' = v_C - v_B = -15 m/s - (+5 m/s) = -20 m/s[/tex]
and the direction will be westward (negative sign).