Found the answer:

P1 is the normal price.

P2 is the promotion price.

X*P1=15 : X purchased at the normal price for $15

(X+3)*P2 = 15 : But you get 3 more, so X+3 times whatever the promotion price is also $15

Now deal with those dozen. A dozen purchased at the promotion price (P2) is $2 less than a dozen purchased at the normal price (P1):

12P1 - 2 = 12P2

Three equations, three unknowns. Solve it with algebra, or substitute in the different answers to check if they work.

In each case use the given x to solve two of the equations, then check those values in the third equation.

If x=15

(x+3)*P2 = 15 substitute 18P2 = 15 divide P2 = 5/6

12P1 - 2 = 12P2 substitute and add 2 to both sides 12P1 = 10+2 = 12 so P1 = 1

Check with the third equation x*P1=15 substitute 15*1=15

This is the correct answer.

Try the others just for fun.