So it can be anything over 8 inches but has to be less than 16.
Answer:
The value of x is reciprocal of the difference of a and b.
Step-by-step explanation:
We are given the equation, 
On simplifying, we get,

i.e. 
i.e. 
i.e. 
So, we get,
The value of x is
.
That is, the value of x is reciprocal of the difference of a and b.
Your answer is 40mm
3+12+12+13
=40
3x+4x+5x=54
12x=54
x=4.5
Plug in the value of x into all three segments.
AB= 3*4.5= 13.5
BC= 4*4.5= 18
AC= 5*4.5= 22.5