A collector has 90 movie posters and 75 band posters. She wants to sell 18 movie posters but still have her poster collection maintain the same ratio of 90:75. If she sells 18 movie posters, how many band posters should she sell?

72:57 is the answer to your question

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.