Concept: The question tests on the basics of Rates(Time and Work)

Solution:

3 Men working 7 hours a day take 10 days

Therefore work done by 1 man working 1 hour a day in 1 day = 1/3710

Work done by 6 men working 8 hours a day in 1 day = 68/3710=8/35

Similarly, 6 Boys working 7 hours a day take 10 days

Therefore work done by 1 Boy working 1 hour a day in 1 day = 1/6710

Work done by 2 Boys working 8 hours a day in 1 day = 28/6710=8/210

 Work done in 1 day when 6 Men and 2 boys work together = 8/35+8/210

=56/210

=4/15

Time taken to do 1 work = 15/4 days

Time taken to do double the work = 215/4

=15/2

=7.5days

Option B