If 3 men or 9 boys can finish a work in 21 days, then in how many days can 5 men and 6 boys finish the same work?

9

3 men or 9 boys can finish a work in 21 days  ( Given )

3 men = 9 boys

1 men = 3boys

Now,

5 men + 6 boys

= 5 x 3 boys + 6 boys

= 21 boys

Total work = 21 x 9           (Time x No. of boys)

Time taken by 21 boys ( 5 men + 6 boys ) to complete the work = \(\frac{21 × 9}{21}\)

= 9 days