The question asked is incomplete, they should ask "what is the sum of different height that stack should have while infix to postfix conversion."
The resultant postfix will be: - PQ+RS+*T/ABC+*+
and the different height will be ,2,3,4,5;
sum is 1 + 2 + 3 + 4 + 5 = 15.
Ans = 15