A solid cylinder of radius R cm and height H cm is melted and recast into smaller identical solid cylinders of height h cm and radius r cm. The number of smaller solid cylinders obtained is :

1. $\dfrac{RH}{rh}$

2. $\dfrac{R^2H}{r^2h}$

3. R²H ÷ r²h²

4. $\dfrac{RH^2}{rh}$

The total surface area of an open pipe of length H cm, external radius R cm and internal radius r cm, is :

1. πR² - πr² + 2π(R - r)H

2. 2πR² - 2πr² + 2πRH

3. 2πRH - 2πrh + πR² - πr²

4. 2(πR² - πr²) + 2πRH + 2πrH

How many cubic meters of earth must be dug out to make a well 28 m deep and 2.8 m in diameter ? Also, find the cost of plastering its inner surface at ₹ 4.50 per sq meter.

What length of solid cylinder 2 cm in diameter must be taken to recast into a hollow cylinder of external diameter 20 cm, 0.25 cm thick and 15 cm long?