A blue puck with a mass of 4.40*10^−2kg , sliding with a speed of 0.250m/s on a frictionless, horizontal air table, makes a perfectly elastic, head-on collision with a red puck with mass x, initially at rest. After the collision, the velocity of the blue puck is 5.00*10^−2m/s in the same direction as its initial velocity. It asks me for both the mass and the velocity of the red puck. Well, since neither mass nor velocity is given to me, should I use P=m*v to find the momentum lost and then transfer it to the red puck? Or do I just derive an equation from momentum conservation and plug that into another momentum conservatio equation so I only have one unknown variable, and solve for that? Please advise. Thanks.