Physics
posted by Trace on .
A copper wire has a resistance of 24 Ω at 20°C. An aluminum wire has three
times the length and twice the radius of the copper wire. The resistivity of copper is 0.6 times that of
aluminum. Both Al and Cu have temperature coefficients of resistivity of 0.004°C–1.
What is the resistance of the aluminum wire at 20 degrees C?
please help me I have no idea what to do. I know that I should use R=p(L/A), but I'm not even sure where to start.

You don't need to use the temperature coefficents of resistivity. You just want the ratio of resistances. That will required knowing the resisitivities of each metal.
R(Al)/R(Cu) = [rho(Al)/rho(Cu)]*[L(Al)/L(Cu)]*[r(Cu)/r(Al)]^2
= (1/0.6)*3*(1/2)^2
rho is the resisitivity. They tell you what the resistivity ratio is 
I think the person to factor in pi as part of th answer because the cross section is a circle.
It should be (1/0.6) (3/pi(2^2))