Using Pythagoras Theorem,
(12)^2+ (a)^2= (13)^2
144+a^2= 169
a^2= 169-144
a^2= 25
a= √25
a= 5mm
Hope it helps you.
Look at the ratio (2:3). You can see that the numbers of "ratio units" add up to 5, and that the total number of books is 100 times that value. Then each "ratio unit" stands for 100 books.
3 ratio units of chapter books mean there are 300 chapter books.
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You would want to add 5.2 to both sides to get 4.2y by itself on one side. Then you would divide both sides by 4.2 to get y by itself. Whatever y equals is the answer. Hope this Helps!