Answer:
While these mounds take many forms and served different purposes, with each in a sense telling its own story, all were built entirely by hand, usually by piling up earth one basket-full at a time. Some served as burial mounds for the honored dead, while other flat-topped mounds were parts of large cities or towns and held temples or ceremonial buildings, and still others were built in the shape of giant animals.
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.
the marginal social benefit of producing and consuming another unit equals the marginal social cost.
