Question

A thread is wound 200 times round a reel of diameter 5 cm. Use the value 3 for it to find the approximate length of the thread in metres.

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Answer 1

Length of the thread = 30 metres

Explanation:

Length of the thread in one round of a reel = Circumference of the reel

C =
`2\pi r`

Since radius of the reel =
`\frac{\text{Diameter}}{2}`

=
`(5)/(2)`

= 2.5 cm

Circumference of the reel = 2(π)(2.5)

= 5π

= 5×3

= 15 cm

Length of the thread wound in one round around the reel = 15 cm

Since, thread is wound 200 times round a reel, length of the thread

= 200 × 15

= 3000 cm

≈ 30 metres

Therefore, Option (B) will be the answer.