# Aptitude - Volume and Surface Area - Discussion

Discussion Forum : Volume and Surface Area - General Questions (Q.No. 5)
5.
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 8 g/cm3, then the weight of the pipe is:
3.6 kg
3.696 kg
36 kg
36.9 kg
Answer: Option
Explanation:

External radius = 4 cm,

Internal radius = 3 cm.

Volume of iron
 = 22 x [(4)2 - (3)2] x 21 cm3 7
 = 22 x 7 x 1 x 21 cm3 7
= 462 cm3.

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

Discussion:
60 comments Page 6 of 6.

Gopal said:   1 decade ago
How to convert gm to kg ?

Manojit said:   1 decade ago
As the thickness of the pipe is 1 cm.....

Anju said:   1 decade ago
Why internal radius is 3?

Sandeep said:   1 decade ago
Because external diameter is 8cm then.

External radius is 8/2=4cm.

Krishna said:   1 decade ago
Why external radius is 4cm how is it ?

Mobi001 said:   1 decade ago
Thats fine. I understood the logic.

Because from both side the 1cm will be deducted.

Mobi001 said:   1 decade ago
If the thickness of pipe is 1 cm the internal diameter should not be 7 cm if yes then internal radius must be 3.5 ?

Divya said:   1 decade ago
Why [4]^2-[3]^2 while calculating volume?

Neelam shelke said:   1 decade ago
Thickness of the pipe is 1cm, and external radius is 4cm, so 4-1=3, thus the internal radius is 3, as per my thinking.

Anupam Tripathi said:   1 decade ago
Why internal radius is 3 cm?

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