I don't know what to do. I asked this yesterday but got no response and I still don't know what to do :(

A child is sitting on a stationary skateboard with a pile of rocks which she throws in one direction in order to make the skateboard travel in the opposite direction. If the rocks each have a mass of 0.70 kg and she can throw them with a speed of 13 m/s relative to the ground, determine the number of rocks she must throw per minute in order to maintain a constant average speed against a 2.0 N force of friction. (Note: because this is a rate, the answer may not be a whole number.)