So here is the answer. Given that both Eric and Mark got a hamburger which is $4 each. Mark got a lemonade and some cotton candy which costs $5.
So this gives them a total of $4, $4, $5, $?(lemonade). Partial sum is $13 without the lemonade. Since Eric got a $2, the total bill was deducted with $2 and the rest of the amount is $14.50. Therefore, the total bill would have been $16.50. So $16.50 minus $13 (partial sum) is $3.50. So the lemonade costs $3.50. Hope this helps.
