Posted by Katherine on Monday, November 8, 2010 at 4:53pm.
let ..
number of textbooks be x
number of library books by y
number of telephone books by z
x+y+z = 36 (#1)
y+x = 2x --- > y = x (#2)
x+z = 3y --- z = 3y - x (#3)
back in #1
x+y+z = 36
x + x + (3y-x) = 36
x+x+(3x-x) = 36
4x=36
x=9
then y=9
z = 18
9 library books
9 textbooks
18 telephone books
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