Question 39108
This problem would be easier to show you on something physical that we could both move, but I'll do my best to explain here:
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A has the coordinate of 0, so start with A: 
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0
A
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Since D is the midpoint between C & E, and D is 6 points to the left of E, we know that there are 12 points between C & E.  We can use place holders for this because we don't have the actual numbers yet:
A ..?.. C...-6pts......D......6pts......E  
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We also know that C is the midpoint between A and E.  Since there are 12 points between C & E, there must be 12 points between A & C:
A............12pts.............C........6pts........D.........6pts.........E  
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Now we just need to add B into this, 5 points to the left of C:
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
A................B...............C....................D....................E  
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B is 7, C is 12, D is 18