2). The assumption is the male and female retio is 100 to 90 when they grow up. Every year there is 10% left over male on the 20s. After 10 year, the people at 20 will not find wife until they reach 21, because the older people will take the girls first. 70-20 is 50 years. That's the reason I say it will take 500 years for someone to die without a wife. Sure it will never be true in the real life regardless what 恶的共产党 will do.