You can solve this problem several ways
if 2 puzzles cost $5.00 then 1 puzzle costs $2.50
the variable would be the number of puzzles, the cost of each puzzle
stays the same
cost= number of puzzles times the price
2,50 (15)= x
$37,50 =x
also if 3 puzzles cost 7.50 then 15 is 5 times that cost
5 (7,50)= $ 37.50
also ratio and proportion
1 puzzle/$2.50= 15 puzzles/ x $
cross multiply
x= $37,50
hope this helps