if 20 men or 30 girls can do a piece of work in 36 days, in how many days will 10 men. and 12 girls do it?

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.

answer; /// i believe that the correct answer is (c)