I am not going to do all the math.
Conservation of momentum, and energy applies
Momentum
A*20+B*10=A*Va + B*Vb
but A=3B
Energy:
1/2 A 20^2+1/2 B*10^2=1/2 A Va^2 + 1/2 B Vb^2
in equation 1, solve for Va in therms of all else, then substiture it in equation 2, and solve the quadratic.