This month, Judy has checked out 20 books from the library, and  of them were biographies. Which expression can be used to determine the number of biographies Judy checked out?

This is the full question:

This month, Judy has checked out 20 books from the library, and 4/5 of them were biographies. Which expression can be used to betermin the number of brographies Judy checked out?

Choices are here:

A. 20 divide by 4/5
B. 20 x 4/5
C. 4/5 divide by 20
D. 5/4 divide by 20

The answer is: the choice B. 20 × 4/5

Explanation:

You are told that the total number of books checked is 20 and that from them the fraction 4/5 were biographies.

A fraction is a ratio, and that ratio is equal to the ratio of biographies to the total number of book.  That is what is called a proportion.

You can write the proportion in this way:

  4            number of biographies checked
----- =   -------------------------------------------------
  5             total number of books checked

The number of biographies checked is the unknown. Call it x

The total number of books checked is 20.

So the proportion is:

 4           x
---- =   ------
 5          20

And applying equallity properties that drives to:

x = 20 * 4/5.

Which is the answer to the question.