Question 1023843
Make a variable for cost of puzzles.

c for cost of puzzles.
Cost of the $5 puzzles plus cost of the $7 puzzles will be c.


x for the cheap puzzle
y for the expensive puzzle
{{{c=5x+7y}}}



Just, "15 puzzles" is not precise enough for any single answer for c.  c can have a minimum value and a maximum value and a few values in between.  For 15 puzzles and no other constraint, there are 15 possible values for c.   x+y=15.