12 men can do a work in 8 days while 4 boys can do it in 40 days. In how many days 9 boys and 3 men together will complete the same work ?

80/7 days

Given,

12 Men x 8 days = 4 Boys x 40 days,    ..(1)

⇒ 3 Men = 5 Boys

⇒ Men : Boys = 5 : 3  (Efficiency)

Putting Efficiency in (1), 

⇒ 12 (5) x 8 = 480 units.      ..(Efficiency × Days = Total work)

⇒ Total efficiency of 9 Boys and 3 Men = 9(3) + 3(5) = 27 +15 =42

⇒ Time required for 9 Boys and 3 Men to complete total work = \(\frac{480}{42}\) = \(\frac{80}{7}\) days.