Question : A one-metre pipe is made with an inner diameter equal to the outer radius. How much material (in cubic mitres) is required to make the pipe, if it can hold $\frac{88}{7}$ cubic metres of water in it?
Option 1: 37.7
Option 2: 35.5
Option 3: 33.3
Option 4: 36.6
Correct Answer: 37.7
Solution :
Let the radius of the inner cylinder be r and the radius of the outer cylinder be R.
Here, 2r = R
In this figure, the amount of water that will flow will be equal to the volume of the inner cylinder.
The volume of the cylinder,
⇒ πr
2
h = $\frac{88}{7}$
⇒$\frac{22}{7}$ × r
2
× 1 = $\frac{88}{7}$
⇒ r
2
= 4
$\therefore$ r = 2
Then R = 4
Materials used to make pipe (volume) = Volume of outer cylinder – Volume of the inner cylinder = (πR
2
h – πr²h)
= πh (R
2
– r
2
)
= $\frac{22}{7}$ × 1 × (16 – 4)
= $\frac{22}{7}$ × 1 × 12
= 37.7 m
3
Hence, the correct answer is 37.7 m
3
.
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