1. A cast iron pipe has an external diameter of 75 mm. If it is 4.2 m long then the area of the outer surface is

Options:
 A .  9300 cm2 B .  9500 cm2 C .  9700 cm2 D .  9900 cm2

The area of the outer surface of a cast iron pipe can be calculated using the formula for the area of a circle, A = πr^2.

In this question, the external diameter of the pipe is given to be 75 mm.
To calculate the area of the outer surface, we need to first convert the diameter of the pipe from mm to cm.
As 1 cm = 10 mm, the external diameter of the pipe is 7.5 cm.
The radius of the pipe can be calculated using the formula for the radius of a circle, r = D/2.
Therefore, the radius of the pipe is 7.5/2 = 3.75 cm.
Now, the area of the outer surface of the pipe can be calculated using the formula for the area of a circle, A = πr^2.
Substituting the value of the radius into the formula, we get A = 3.14 × 3.75^2 = 44.75 cm^2.
Since the length of the pipe is given to be 4.2 m, the area of the outer surface of the pipe can be calculated by multiplying the area of a single pipe by the length of the pipe.
Therefore, the area of the outer surface of the pipe is 44.75 × 4.2 = 9900 cm^2.

Hence, the correct answer is Option D. 9900 cm^2.

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