Initial total momentum = pi = (m1 + m2)u1
= (20 + 200)(36 x 5/18)
= 220 x 10 = 2200 kg ms-1
Final total momentum = pf = m1v1 + m2v2
= 20(v2 - 4) + 200 v2 [(By using v1 = v2 - 4)]
pf = (200 v2 - 80)
Using the law conservation of total momentum, i.e., pi = pf
We have 2200 = (220 v2 - 80)
v2 = 2280/220 = 10.38 ms-1 = 25.9 m