please help with physics, i keep getting -3.8 and 4?

Question

Ball A, which has mass 23 kg, is moving horizontally left to right at +7.8 m/s when it overtakes the 67 kg ball B, which is moving left to right at +4.7 m/s, and they collide elastically. (a) What is ball A's velocity after the collision? (b) What is ball B's velocity?

for (a) i keep getting -3.81 and for (b) i keep getting 4
what am i doing wrong?

Answer 1

When I use the formula
v1=(m1-m2)*u1/(m1+m2) + 2*m2*u2/(m1+m2)
I get v1 = n1 m/s + n2 m/s. For n1, I get -3.81 which is what you get for a final answer.

When I use the formula
v2 = 2*m1*u1/(m1+m2) + (m2-m1)*u2/(m1+m2)
Same thing happens - the first of my 2 terms is your final.

If the 2nd object has zero velocity (u2=0), the 2nd term drops out. You must be using that version of the equations. But that's not the case here. Both balls are moving (u2=4.7 m/s).