By what factor would the myelin need to decrease membrane conductance (gm) if you wanted conduction velocity in a 10 μm myelinated mouse axon to be 100 times faster than in a 10 μm unmyelinated mouse axon? 

This is a question from a homework set. I understand WHY myelin decreases membrane conductance, and WHY decreasing membrane conductance would increase conduction velocity. I just do not understand how to calculate what factor specifically the myelin would need to decrease membrane conductance by.

The conduction velocity is inversely proportional to the membrane time constant $\tau_m$ and proportional to the length constant $\lambda$. Membrane time constant is proportional to membrane resistance (inverse of membrane conductance), and the length constant is proportional to the square root of membrane resistance.

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