$A$ cylinder of a material is $10\, cm$ long and has a cross-section of $2\, cm^{2}$. If its resistance along the length is $20\, \Omega$,what will be its resistivity in number and units?

(A) The formula for resistivity is $\rho = \frac{R \cdot A}{L}$.
Given:
Resistance $(R)$ = $20\, \Omega$
Area of cross-section $(A)$ = $2\, cm^{2}$
Length $(L)$ = $10\, cm$
Substituting the values into the formula:
$\rho = \frac{20 \times 2}{10} = \frac{40}{10} = 4\, \Omega \cdot cm$.
Thus,the resistivity is $4\, \Omega \cdot cm$.