# 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.

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.
(2)

Roney said:   4 years ago
@Nazim.

From both side the 1cm will be deducted.
so dia = 6cm radius 3cm.
(2)

Dhinesh said:   6 years ago
Because, the VOLUME OF HOLLOW CYLINDER FORMULA is π(R^2-r^2)h.
Assume, (hollow iron pipe is in the shape of a hollow cylinder).
Here, we require RADIUS, but in question, it is in DIAMETER.
Outer dia=8cm[R=d/2=4cm];;then thickness=1cm(I.e, reduce diameter Of outer pipe by 2cm)..since thickness is both sides of pipe..(thickness = 1+1)
This gives diameter Of inner pipe = 6cm[r=d/2=3cm].
Then substitute R&r in formula....
Volume = 22/7 * (4^2 - 3^2) * 21 = 22 * (16 - 9) * 3 = 22*(7)*3 = 462 cm3.
Weight = density * volume.
= 8 * 462 = 3696 g.
= 3696/1000 = 3.696 kg.
(1)

Nazim Shaik K said:   4 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)

Ravi said:   7 years ago
Surface area of iron cube of side 10cm is?

Dhana reddy said:   6 years ago
Why 4^2 -3^2? Please explain this step.

Indranath said:   6 years ago
Why not (4-3)^2?

Gautam said:   6 years ago
Isn't weight=mass*gravity?

Ajay said:   6 years ago
Could you explain me about pi?

Utham said:   6 years ago
The Internal radius and external radius. given is 8g/cm^3 not for hollow space.So, the first find out the full volume that is with the use of external radius. Now subtract hollow space that volume with internal radius. R^2h*r^2h.