A hollow cylinder is made up of metal. The difference between outer and inner curved surface area of this cylinder is 352 cm$$^2$$. Height of the cylinder is 28 cm. If the total surface area of this hollow cylinder is 2640 cm$$^2$$, then what are the inner and outer radius (in cm)?

Let say, outer radius is R and inner radius is r .

So, According to question :

$$2\pi\ \left(R-r\right)\times h\ =\ 352\ .$$

or, $$\left(R-r\right)=\ \frac{352}{2\times\ \frac{22}{7}\times28}=2\ ............................\left(1\right)$$

We know that , Total surface area of hollow sphere is = $$2\pi\left(R+r\right)\left(h+R-r\right)\ .$$

So, $$2\pi\left(R+r\right)\left(h+R-r\right)\ =\ 2640\ .$$

or, $$2\pi\left(R+r\right)\left(28+2\right)\ =\ 2640\ .$$

or, $$\left(R+r\right)=\ \frac{2640}{2\times\frac{22}{7}\times30}=14\ ....................\left(2\right)$$

So, By solving (1) & (2) ,we get  :

$$R=8\ and\ r=6\ .$$

D is correct choice.