Answer:
She should sell <u>66</u> band posters.
Step-by-step explanation:
Given:
A collector has 135 movie posters and 90 band posters.
The collector wants to sell 36 movie posters.
But still want her collection to be the same ratio of 135:90.
If she sells 36 movie posters.
Now, to find the band posters she should sell.
Her current ratio of movie posters to band posters is 135:90.
If she sells 36 movie posters, she will have left.

As, she wants the ratio to be maintain same.
So, let the number of the band posters she should sell be 
<em>If, 135 is equivalent to 90.</em>
<em>Thus, 99 is equivalent to </em>
<em />
Now, to solve using cross multiplication method:

<em>By cross multiplying we get:</em>
<em />
<em />
<em>Dividing both sides by 135 we get:</em>
<em />
<em />
<u><em>The number of the band posters she should sell = 66.</em></u>
Therefore, she should sell 66 band posters.