Posted by Katherine on Wednesday, November 10, 2010 at 5:02pm.
Do it this way
a=library books
b=textbooks
c=telephone books
You know a + b + c = 36
You know that a + b = 2b (library books plus textbooks equals twice number of textbooks)
You know that b + c = 3a (textbooks plus telephone books equals three times number of library books)
Take equation #2 a+b=2b
Subtract b from each side
a=b The number of library books equals the number of textbooks
Take equation #3 b+c=3a Substitute a for b since you know now from equation #2 that a=b Now you have a+c=3a
Subtract a from each side c=2a
Now, go to equation #1 a+b+c=36
You know a=b
You know c=2a
substitute
a + a + 2a = 36
Combine like terms and divide
4a = 36 a= 9
Again, you know a = b and c= 2a therefore a=9, b=9, c=18
9 library books
9 text books
18 telephone books
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