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Let a, b and c be x-coordinates of the three vertices.
You are given that a + b + c = 10.
Midpoints have x-coordinates 0.5(a+b), 0.5(a+c) and 0.5(b+c).
When you add them, you will get
0.5(a+b) + 0.5(a+c) + 0.5(b+c) = a + b + c = 10,
i.e. the same 10 units as your answer.