Question

A rugby player of mass 75 kg, running east at 8.0 m/s, tackles another player of mass 90 kg who is running directly towards him at 5.0 m/s. If the two players cling together after the tackle, what will be their common velocity?

Answer 1

You can use the formula:
(m1 + m2)vf = m1vi
m1 = mass of first rugby player (75 kg)
m2 = mass of second rugby player (90)
vf = their final velocity
vi = the initial velocity 

(m1 + m2)vf = m1vi
(75 + 90)vf = (75)(8.0)
165(vf) = 600
        vf = 3.64 m/s

hope this helps :)