Answer
Use the conservation of momentum principle. The momentum before the collision equals the momentum after the collision.
m1u1+m2u2=m1v1+m2v2
This implies:
$$(40 kg)(0) + (15 kg)(3.0 m/s) = (40.0 kg + 15 kg) v$$
$$v = 0.8 m/s$$
Work Step by Step
$$(40 kg)(0) + (15 kg)(3.0 m/s) = (40.0 kg + 15 kg) v$$
$$v = 0.8 m/s$$