Let the length of segment AB be x.
BC then is AB + 6 or x + 6.
AC is AB + BC - 13 or x + x + 6 - 13 = 2x - 7
The sum of the three lengths is 78.
x + x + 6 + 2x - 7 = 78
4x - 1 = 78
4x = 79
x = 19.75
BC = x + 6 = 19.75 + 6 = 25.75
Answer: B. 25.75 mm
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