At Halloween, a shopkeeper bought six boxed surprises. One box contained treats, while the other five contained horrible tricks. Written clearly on the boxes were these prices: £15, £31, £19, £20, £16 and £18.

He kept the box of treats for himself, but sold all five trick boxes to two customers called Morbid and Cruella.

The total price of the boxes bought by Morbid came to exactly twice the total price of the boxes bought by Cruella.

What was the cost of the box containing the treats?