Question

# If light can travel 31 times around the world in 4 seconds, how many times can it circle the world in 10 seconds?(A). 124 times(B). $66\dfrac{1}{2}$times(C). $82\dfrac{1}{2}$times(D). 77.5 times

Hint: Start by finding out the rate or cost of each kg or 1 kg of rice . Later this value or rate can be used to find any quantity of rice , which can be computed by multiplying the rate and the desired quantity . All the units of different quantities must be written correctly.

Given, No. of times round taken in 4 seconds = 31
Now, we know it takes 4 sec for 31 rounds, so let’s try to find out how much round would have been made in 1 sec.
Therefore, No. of times round taken in 1 second =$\dfrac{{31}}{4}$
Now that we know the number of rounds in 1 sec , let us compute the number of rounds made in 10 seconds as per the question.
the number of rounds made in 10 seconds will be found out by the product of time(sec) and number of rounds made in 1 sec.
So, number of rounds made in 10 seconds =$10 \times \dfrac{{31}}{4} = 77.5$
Therefore, light makes 77.5 times around the world in 10 seconds.
Hence option D is the correct answer.

Note: The methodology applied here is known as Unitary method , In this method we try to find value of each unit in terms of other units and later that each unit value can be used for vast scale of computation .
Such similar questions can be asked the other way round. For e.g. how much time it takes for light to complete 62 rounds ,if it takes 4 sec for 31 rounds.
Then , we’ll find out time taken for 1 round = $\dfrac{4}{{31}}\sec$
Now , time taken for 62 rounds will be = $\dfrac{4}{{31}} \times 62 = 8\sec$
So , it takes 8 sec for 62 complete rounds around the world.