Answer: Option C
Let's assume that the son's speed is x units per day.Then, the father's speed is twice as fast, which means his speed is 2x units per day.
Now, let's consider the job that needs to be completed. Let the total work be W.The time taken by the father to complete the job alone can be calculated using the formula:
time taken = work / rate
where rate is the speed at which the job is done.
So, the father's time taken to complete the job alone is:
W / (2x)
Similarly, the time taken by the son to complete the job alone is:
W / x
Now, let's consider the time taken by the father and son together to complete the job.We know that they can complete the job in 12 days.Using the formula:
work done = rate * time taken
we can write:
W = (2x + x) * 12
Simplifying this equation, we get:
W = 36x
Now, we can substitute the value of W in the above equations to find the time taken by the father and son to complete the job alone.
Time taken by the father alone:
W / (2x) = (36x) / (2x) = 18 days
Time taken by the son alone:
W / x = 36 days
Therefore, the correct answer is option C, 36 days.
To summarize:

• Let x be the son's speed and 2x be the father's speed.
• Let W be the total work that needs to be done.
• Father's time taken to complete the job alone: W / (2x)
• Son's time taken to complete the job alone: W / x
• Using W = 36x, we get father's time taken = 18 days and son's time taken = 36 days.

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