Please help with this math question

Two particles P and Q have masses km and 3m respectively,
where k < 3. The particles are connected by a light inextensible string which passes over a smooth light fixed pulley. The system is held at rest with the string taut, the hanging parts of the string vertical and with P and Q at the same height above a horizontal plane, as shown in the diagram. The system is released from rest.
After release, Q descends with acceleration 1/3g.

a Calculate the tension in the string as Q descends.
b Show that k = 1.5.
c State how you have used the information that the pulley is smooth.


After descending for 1.8s, the particle Q reaches the plane. It is immediately brought to rest by the impact with the plane. The initial distance between P and the pulley is such that, in the subsequent motion, P does not reach the pulley.
d Show that the greatest height, in metres, reached by P above the plane is 1.26 g.


I understand a, b and c but for question d I always end up getting 18/25g as my final answer.