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 B