M = mass of the girl = 50 kg
m = mass of skate = 10 kg
V' = initial velocity of girl and skate together before the girl jump = 2 m/s
V = velocity of girl after the jump relative to ground = 3 m/s
v = velocity of the skate after girl jumps = ?
using conservation of momentum
(M + m) V' = MV + m v
inserting the values in the above equation
(50 + 10) (2) = (50) (3) + 10 v
v = - 3 m/s
