A coin is in a fixed position. Another identical coin is rolled around the edge of the first one. How many complete revolutions will be made by the revolving coin before it reaches its starting point?
Sub Grade: 10

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answer is B.

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Detailed Solution

Suppose we had both coins rotating in opposite directions to each other .
Then each cion would have had to rotate 360 degree .
Hence if we fix one of the coins one has to rotates 720 degree or 2 complete revolutions
This can also be looked at in the following way
Suppose we had a line of length 2𝝿r to cover which a coin would require one complete revolution.
Also if it is slid along that line that accounts for no revolution whereas sliding along a circle would account for 1 more revolution .Hence there area 2 complete revolutions .
Type Exam: CBSE

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