A 110 kg tackler moving at 2.5m/s meets head on (and holds on to) an 82 kg halfback moving at 5.0m/s. what will be their mutual speed immediately after the collision?!

The situation in the problem is called an inelastic collision where kinetic energy is not conserved only momentum is conserved. We do as follows:

m1v1 + m2v2 = (m1 + m2) vf

vf = (m1v1 + m2v2) / (m1 + m2)

vf = (110) (2.5) + (82) (5.0) / (110 + 82)

vf = 3.57 m/s