



 
Hi Julie, If each books is n inches thick and there are x books that's a total of xn inches which you know is 36 inches. Hence
If each book is half an inch thinner they are each n  1/2 inches thick. That would allow for x + 6 books on the shelf. The shelf is 36 inches long so
By the first equation n = 36/x. Substitute this into the second equation and solve for x. I hope this helps,  


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