I don’t really get your question, because you said that ¾ of them are girls, Then how many girls are working in the piano?
You did not stated the question well.
However, I will try to answer this base on my understanding.
There are a total of 12 students that works in the library.
IF ¾ of them are girls:
=> 12 / 4 = 3 *3 = 9
=> 9 of them are girls and 3 of them are boys.
Thus, how many girls are working in the piano.
The probability is 9, there might be 9 girls working in the piano, or less than 9. Since you did not mention how many girls worked in different part of the library.
