Judging on the question I figured that to find the amount of money she would have to pay you would need to figure out the amount payed for one album alone.
To do this you need to divide $18 by 5. Your answer for this should be $3.60/per album.
You then want to multiply the amount per album, $3.60, by the amount his sister wants to pay, 2.
Your answer should be $7.20
Hope this is correct!
