Question

这道奥数题怎么做？详细点

In a certain town some people were affected by a ’flu’ epidemic. In the first month 20% of the population contracted the flu whilst 80% were healthy.
In the following month 20% of the sick people recovered and 20% of the healthy people contracted the disease.
What fraction of the population is healthy at the end of the second month?

Answer 1

在一座城里一些人受流感的影响。在第一个月20%的人口感染了流感仅有80%是健康的。
在接下来几个月20%的病人恢复和20%的健康的人感染疾病。　
那么第二个月结束有多少人是健康的？
第一个月结束：20%患病，80%健康
第二个月结束：康复的人：20% x 20%=4%
依旧健康的人：（1-20%）x 80%=64%
所以第二个月结束健康的人有：64%+4%=68%

Answer 2

80%*80%+20%*20%=0.68=68%

Answer 3

80%*（1-20%）+20%*20%=68%