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How long will John take to make a round of a circular field of radius $21$ m cycling at the speed of $8$ km/hr?

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40 seconds

59.4 seconds

85 seconds

15 seconds

answer is B.

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Detailed Solution

It is given that radius of a circular field is

21

m. John is cycling in the field with the speed of

8

km/hr.

\Rightarrow r = 21

m
We need to find the time taken by John to round the field.
Here the distance travelled by John to round the field is equal to the circumference of the circular field.
Distance travelled = Circumference
We know that the circumference of a circle is

2 πr

units, where

r

is its radius.
Therefore,

Distance travelled = 2 πr

\Rightarrow Distance travelled = 2 \times π \times 21

\Rightarrow Distance travelled = 2 \times \frac{22}{7} \times 21

\Rightarrow Distance travelled = 2 \times 22 \times 3

\Rightarrow Distance travelled = 132

\Rightarrow Distance travelled = 0.132

km [because 1000m=1km]
We know that,

Time = \frac{Distance travelled}{Speed}

\Rightarrow Time = \frac{0.132}{8}

\Rightarrow Time = 0.0165

hours

\Rightarrow Time = 0.0165 \times 3600

seconds [because 1 hour = 3600 seconds]

\Rightarrow Time = 59.4

seconds
Therefore, John takes

59.4

seconds to round the field.
Hence, the correct option is 2.

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