A hollow iron pipe is 21 cm long and its external diameter is 8 cm. If the thickness of the pipe is 1 cm and iron weighs 8g/cu.cm, then the weight of the pipe is

External radius = 4 cm 

Internal radius = 3 cm 

Volume of iron =  $\frac{22}{7}\times 42-32\times 21cm3$=  $462cm3$   

Weight of iron = (462 x 8)gm = 3696 gm = 3.696 kg