4 boys or 7 girls can complete a piece of work in 18 days. How many days will 12 boys and 21 girls together take to complete this job?

3 days

4 Boys = 7 Girls

Boys : Girls = 7 : 4  (Efficiency)

Total work = 4(7) x 18 days = 504 units,

⇒ Time required by 12 Boys and 21 Girls to complete total work = \(\frac{504}{12(7) + 21(4)}\) = \(\frac{504}{168}\) = 3 days,   ..(\(\frac{Work}{Efficiency}\) = Time )