STRENGTH OF THE ARMY

An army of $N$ soldiers is present on the field, the ith soldier has $A[i]$ power. Colonel wants to arrange the soldiers in a forces such that size of each force should be between $X$ to $Y$ and each soldier can be present in $at$ $most$ $one$ $force$. Let's say arranged forces are $F(1), F(2), ...., F(M)$. Then Strength of the army can be calculated as sum of the average power of each fo