# 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
Explanation:

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 1 of 6.

Debosmita Misra said:   3 months ago
The external Diameter is given 8 cm and the thickness of the pipe is given 1 cm.
Hence, the External radius (R) would be 4 cm & Internal radius(r) would be (8 - 1 - 1)/2 = 3cm.

And the volume will be PI*(R*R - r*r)*h = (22/7)*(4*4 - 3*3)*21 = (22/7)*(16-9)*21 = 462 g.
Therefore, weight of the pipe = (462*8) g = 3696 g = 3.696 kg.

Bala said:   6 months ago
@Dinesh.

Thank You for your good explanation.

Dinesh Reddy said:   11 months ago
Can anyone explain this answer once again deeply?

Pema said:   2 years ago
Why cant it be =pi * 3^2 * 21?

Ram said:   2 years ago
External diameter = 8 cm.
External radius = R =4 cm.
Given Thickness= 1 cm.

So, internal diameter= (8-1) cm = 7 cm.
Internal radius = r = 3.5 cm not 3 cms.
According to the volume of iron;

= 21*(22/7)*(16-12.25),
= 247.5 cm3.

Weight of iron = 247.5 * 8 = 1980 gram or 1.98 kg.
(1)

Roney said:   3 years ago
@Nazim.

From both side the 1cm will be deducted.
so dia = 6cm radius 3cm.
(1)

Nazim Shaik K said:   3 years ago
External diameter = 8 cm.
External radius = R =4 cm.
Given Thickness= 1 cm.

So, internal diameter= (8-1) cm = 7 cm.
Internal radius = r = 3.5 cm not 3 cms.

According to the volume of iron;

= 21*(22/7)*(16-12.25),
= 247.5 cm3.

Weight of iron = 247.5* 8 = 1980 gram or 1.98 kg.

Sunil said:   4 years ago
Why 4 square minus 3 square equals to 7 * 1.

Rahul said:   4 years ago
There is a formula for hollow sphere 3.14 (R^2-r^2)h.

CHINMAYE said:   5 years ago
@Rohan.

It's given in the question that iron weighs 8 g/cm3. It's nothing but a density.