Question 1023843
Let y = the total cost, and x = the number of $5 puzzles.

==> 15 - x = the number of the $7.50 puzzles

==> Total cost = y = 5x + 7.50(15-x) = 112.50 - 2.50x, or

{{{highlight(y = 112.50 - 2.50x)}}},

where x = 0,1,2,3,4,...,15.