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Question 773824:  Points A,C, and B are collinear. Point C is between A and B. AB=15. Find AC and BC if AC is twice as long as BC. 
 Answer by josgarithmetic(39630)      (Show Source): 
You can  put this solution on YOUR website! You'll need a drawing to see the parts clearly, but you have this:
 
 
AB is 15 as given. 
x = AC 
y = BC 
x=2y according to description and simple substitution.
 
ALSO, x+y=15.
 
 
Again simple substitution will give you a simple one-variable equation: 
x+y=15 
2y+y=15 
3y=15 
Divide both sides by 3, 
y=5.  This means x=1*5=10.
 
 
ANSWER---------------------- 
AC = 10  and BC =5 
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