After collision with the wall, speed of Q is e.8u/9
So P and Q will collide, P has velocity 5u/9, and Q : -e.8u/9
After collision of P and Q again, Q will have velocity V
2mu. 5/9 – eum.8/9 = mV
2u. 5/9 – eu.8/9 = V
and
V/(5u/9 - -8e/9) = 1/3
3V = 5u/9 + 8eu/9
==> 3[2u. 5/9 – eu.8/9] = 5u/9 + 8eu/9
30 – 24e = 5 + 8e
e = 25/32
c) as Q is moving towards the wall, then it will collide, so it will reverse it motion, i.e. go back to P, as P is at rest then they will collide!