There are 12 students working in a library if three fourth of them are girls how many girls are working in the piano

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.