Let his and skateboard's combined mass is x we know F1=-F2 0.5kg*10m/s=-0.05m/s xkg 5=0.05x(minus cancel with the m/s as it represented the opposite direction of velocity and now there is no velocity in this equation.. so minus is avoidable. and the kgs cancel out) x=10 so their combined mass is a 100 kg.. (I hope I didn't mess thing up for you)
