Solution:
Let the work done by Rahul, Rakshita, and Gurmeet be a,b, and c units per day, respectively, and the total units of work are W.
Hence, we can say that 7(a+b+c)<W (Rahul, Rakshita, and Gurmeet, working together, would have taken more than 7 days to finish a job).
Similarly, we can say that 15(a+c)> W ( Rahul and Gurmeet, working together would have taken less than 15 days to finish the job)
Now, comparing these two inequalities, we get: 7(a+b+c)<W<15(a+c)
It is also known that they all worked together for 6 days, followed by Rakshita, who worked alone for 3 more days to finish the job. Therefore, the total units of work done is: W=6(a+b+c)+3b
Hence, we can say that 7(a+b+c)<6(a+b+c)+3b<15(a+c)
Therefore, (a+b+c)<3b⇒a+c<2b, and 9b<9(a+c)⇒b<a+c
⇒a+b+c<3b⇒7(a+b+c)<21b and 15b<15(a+c)
Hence, The number of days required for b must be in between 15 and 21 (both exclusive).
Hence, the correct option is D
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