If 3 men and 4 boys can do a piece of work in 8 day, then 6 men and 8 boys can do the same work in?

Let the men do the work in a days and the boys in b days.
∴ (3/a) + (4/b) = 1/8 ..... (i)
Now, 6/a + 8/b = 2[(3/a) + (4/b)] = 2 x 1/8 = 1/4
So, 6 men and 8 boys can do the same work in 4 days.