Explanation:
20 men can do 1 W in 36 days
=> 20 men can do 1/36 W in 1 day
=> 10 men can do 1/72 W in 1 day.
30 girls can do 1W in 36 days
=> 30 girls can do 1/36 W in 1 day
=> 12 girls can do 1/90 W in 1 day.
Together 10 men and 12 girls can do 1/72 + 1/90 W in 1 day
= 9/360 = 1/40 W in 1 day.
Therefore they need 40 days to do the piece of work.
