Answer:
31 km/h in the direction which the bus is travelling
Explanation:
ball rolls at a speed of 4 km/h towards the bus back. Thus, V_bb = 4 km/h
Velocity of the bus relative to the ground; V_bg = 35 km/h
Let v_br be velocity of the ball relative to the ground.
If we define the direction to be positive, we have;
v_br = V_bg - v_bb
v_br = 35 - 4
v_br = 31 km/h in the direction which the bus is travelling
