A girl riding a b

Let m₁ and m₂ be the mass of girl with bicycle and stone respectively i.e. m₁=50kg & m₂=0.5kg

Also we have given u₁=5m/s, u₂=5m/s and v₂=15 m/s and we have to find v₁

As we know by law of conservation of momentum

initial momentum (girl, bicycle, stone) =final momentum (cycle and girl) + stone

i.e.

Therefore, the speed of cycle and girl decreased by