question_answer

Refer to Design B

A boy is playing with brooch B. He makes revolution with it along its edge. How many complete revolutions must it take to cover 80 K mm ?

A) 2

B) 3

C) 4

D) 5

Correct Answer: C

Solution :

Directions (iii-iv) We have, circumference of silver part = 44 mm \[\therefore \,\,\,2\pi r=44\] \[\Rightarrow \,\,\,\,r=\frac{44}{2\times \frac{22}{7}}=7\,\,mm\] \[\therefore \,\,\,\,R=r+3=7+3=10\,\,mm\] Required number of revolutions \[=\frac{Distance\text{ }covered}{Circumference}\] \[=\frac{80\pi }{2\pi R}=\frac{80\pi }{2\pi \times 10}=4\]