Well, the ratio is 90:75, which can be simplified to 18:15. If she sells 18 movie posters, she will have 72 left. Now since the ratio is 18;15, and she sells 18 movie posters, she has to sell 15 band posters to make the ratio 72:60. Since 18 goes into 72 4 times and 15 goes into 60 4 times, she has to sell 15 band posters to keep the ratio the same.
