**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 = 1/3∗7∗10

Work done by 6 men working 8 hours a day in 1 day = 6∗8/3∗7∗10=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 = 1/6∗7∗10

Work done by 2 Boys working 8 hours a day in 1 day = 2∗8/6∗7∗10=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 = 2∗15/4

=15/2

=7.5days

Option B