a 30-kg girl and a 25-kg boy face each other on Friction-free roller blades. the girl pushes the boy, who moves away at a speed of 1m/s. what is the speed of the girl?

how do i calculate this?

Use conservation of momentum: m1*v1+m2*v2 = 0 (assuming they were at rest to start with).

So: 30*v1 + 25*1 = 0;

Find v1 (the velocity of the girl) from this.