12 men or 20 boys can finish a work in 10 days. What part of the same work will be done by 4 men and 4 boys in 4 days?

$\frac{16}{75}$

Formula to be used here;

${ M}_{1 }$ x ${ D}_{1 }$ = ${ M}_{2 }$ x ${ D}_{2 }$,

⇒ 12M x 10 = 20B x 10

⇒                M  :  B

Efficiency     5  :   3

⇒ Total Work = 12 (5) x 10 = 600 units.

⇒ Work done by 4 men and 4 boys in 4 days = $\frac{W}{32}$ = 4 

⇒ W' = 128 units

⇒ W' is % of total work =  $\frac{128}{600}$ = $\frac{16}{75}$.