Chef and Icecream

Chef owns an icecream shop in Chefland named scoORZ. There are only three types of coins in Chefland: Rs. 5, Rs. 10 and Rs. 15. An icecream costs Rs. 5. There are $N$ people (numbered $1$ through $N$) standing in a queue to buy icecream from scoORZ. Each person wants to buy exactly one icecream. For each valid $i$, the $i$-th person has one coin with value $a_i$. It is only possible for someone