Assuming the question is : "how much does 1 box of blueberries cost?"
After he bought 5 boxes of blueberries and 3 watermelons, he was left with $16.50
Therefore, 5 boxes of blueberries and 3 watermelons cost $60.50 - $16.50 = $44
We are given the cost of 1 watermelon is $8.30
Therefore, 3 watermelons cost $8.30 x 3 = $24.90.
-> 5 boxes of blueberries cost $44 - $24.90 = $19.10
-> 1 box of blueberries cost $19.10 : 5 = $3.82