Hi there! :)
Answer:
In order to maintain the same ratio, she should sell 18 band posters.
Explanation:
Initial ratio:
Ratio is: 105 movie posters for 90 band posters = 105 : 90
She sells 21 movie posters, which means that we are left with 105 movies minus the 21 she sold.
105 - 21 = 84 → She now has 84 movie posters.
In order to answer your question, you'll need to use the cross product method:
105 movie posters → 90 band posters
84 movie posters → X
(90 × 84) ÷ 105 = X
7,560 ÷ 105 = X
72 = X ⇒ Number of band posters she should have left to maintain the same ratio.
She has 90 band posters :
90 - X = 72
Add "X" to each side of the equation → 72 + X = 72 + X
90 = 72 + X
Subtract 72 from each side of the equation → 90 - 72 = 18
18 = X
There you go! I really hope this helped, if there's anything just let me know! :)