Given: mass of boy = 40 kg = mb, mass of skateboard = ? = mk, speed of skateboard vx = - 20 m/s, speed of boy vy = 1.5 m/s
Both the skateboard and the boy were initially at rest.
The total momentum before and after the jump must be equal.
0 = mb x vy + mk x vx
0 = 40 x 1.5 + mk x (-20)
0 = 60 - 20mk
20mk = 60
mk = 6020 = 3 kg.
