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