A hollow cylindrical tube 20 cm long is madeofiron andits external and internal diameters are 8 cm and 6 cm respectively. The volume (in cubic cm) of iron used in making the tube is (Take 
)
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 440
volume of hollow cylinder = π(r12 − r22)h
h = 20
r1 (external radius) = external diameter ÷ 2 = 8 ÷ 2 = 4
r2 (internal radius) = internal diameter ÷ 2 = 6 ÷ 2 = 3
volume of hollow cylinder = π(42 − 32)20 = 
(7)20 = 440
Short Trick:
In this question answer will be multiple of 11. So check the option which will be divisible by 11.
Only option 2 is divisible by 11.