The electrical resistance of a wire varies directly as its length and inversely as the square of its diameter. If a wire 30 meters long and 0.75mm in diameter has a resistance of 25 ohms,find the length of a wire of the same material whose resistance and diameter are 30 ohms and 1.25mm, respectively

Combined variation solution please
The answer is 100 I have no solutions of it



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The combined variation equation is:

Resistance (ohms) = k (Length)

a.  Find the constant, k.
b.  Solve for length of the same material.

Please see my handwritten solution.  Notice the progression in units of measure.   You may follow this step in solving similar problems.  

Hope this helps :-)
1 5 1
Or you can convert diameter (in mm) to meter, although it's the same dilemma, that is, dealing with consecutive zeros. :-)