Determine the resistance of metal tube in term of the external diameter D, the internal diameter d , the length l and the resistivity. Calculate the resistance of a copper tube 0.5cm thick and 2m long. The external diameter is 10cm

The current flowing in a wire is inversely proportional to the resistance of the wire. If the current is 5 amperes (A) when the resistance is 24 ohms ($\mathrm{\Omega }$), for what resistance will the current be 8 amperes?

What is the electric current produced when a voltage of 9V is applied to a circuit with a resistance of $99\mathrm{\Omega }$?

Coaxial cable has radius a of copper core and radius b of copper shield. Between there is an insulator with specific resistivity $\zeta$. What is the resistance of this cable with length L between the core and the shield?
First, I tried to solve this like this: $dR=\zeta \frac{l}{S}$
In our case the length is dr, and therefore I suppose that the area of this ring is 2πrdr: $dR=\zeta \frac{dr}{2\pi rdr}$ The solution sheet says: $dR=\zeta \frac{dr}{2\pi rL}$
I know that something is wrong with my equation, because dr goes away and then I cannot integrate from a to b. But why is in the solution L instead of dr? As I understand problem instruction L= b-a. And therefore L=dr which doesn't make sense to me.

How much power is produced if a voltage of 2V is applied to a circuit with a resistance of $10\mathrm{\Omega }$?

What is the resistivity coefficient of graphene?

What is the statement of Ohm's Law?