There are 12 paperback mysteries on a shelf. This is 40% of the books on the shelf.
How many total books are on the shelf?
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There are 30 books in the shelf.

Step-by-step explanation:

To solve this problem we can use a rule of three in such a way that 12 paperback mysteries are related to 40% of the books in the shelf in the same proportion that "x" total books are related to 100% of the books in the shelf. So we have:

12 -> 40%

x -> 100%

40*x = 12*100

40*x = 1200

x = 1200/40

x = 30 books

There are 30 books in the shelf.

There are 30 books on the shelf.

Step-by-step explanation:

12 paperback mysteries are on the shelf. And this is 40% of the books on the shelf.

Let the number of books on the shelf be represented by x. So that:

        40% of x = 12

        × x = 12

         =  12

Multiply both sides by 100, to have;

     40x = 1200

Divide through by 40,

     x =

     x = 30

The number of books on the shelf is 30.