Big al stands on a skateboard at a rest and throws a 0.5-kilogram rock at a velocity of 10.0 m /sec. big al moves back at 0.05 m/sec. what is the combined mass of big al and the skateboard?

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)

P