4a. In a book store, for every 9 books sold, 13 magazines were sold. If they had a 572 sales, how many of the sales were for books? ​

234 books.

Step-by-step explanation:

You need to set up a proportion. Be careful to note what it looks like and the explanation that follows.

9 books            13 magazines

    =    

x                        572 - x                Cross multiply

What you want is the number of book sales from the proportion.

However all you have is the ratio of books to magazines. So you have to set up the total sales to magazines, which is a bit awkward.  572 - x is the number of magazine sales.

9*(572 - x) = 13x                        Remove the  brackets on the left.

5148- 9x = 13x                           Add 9x to both sides.

5148 = 13x + 9x                         Transpose and add the right.

22x = 5148                                Divide by 22

x = 5148/22

x = 234                    

234 books were sold.

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