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A hollow cylindrical tube 20 cm long is made of iron and its external diameter is 8 cm. The volume of iron used in making the tube is 440 cm3. What is the thickness of the tube? 

(a) 1 cm 

(b) 0.5 cm 

(c) 2 cm 

(d) 1.5 cm

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Best answer

(a) 1 cm

Let the internal radius of the tube be r cm. 

External radius = \(\frac82\) cm = 4 cm, 

Height = 20 cm 

∴ Volume of iron used in the tube = π(42 - r2) x 20 cm

Given, π x (16 - r2) x 20 = 440

⇒ \(\frac{22}{7}\times20\times(16-r^2) = 440\)

⇒ 16 – r2 = 7 ⇒ r2 = 16 – 7 = 9 ⇒ r = 3

∴ Thickness of tube = 4 cm – 3 cm = 1 cm.

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