If we take the forward direction in which the crate is being pushed to be the positive direction, then the net force on the crate is
∑ F = 250 N + 180 N - 20 N
The two friends push in the same direction so their contributions are positive. Friction opposes this movement and points in the opposite direction and so is negative. Add up the forces on the right side and the net force is
∑ F = 410 N
