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)