求一道ds数学题。

captainer

of the companies surveyed about the skills they requird in prospective employees, 20 percent rquired both computer skills and writing skills, what percent of thecompanies surveyed rquired neither computer skills nor writing skills?
1.of those companies surveyed that required computer skills, half required writing skills
2.45 percent of the companies surveyed required writing skills but not computer skills
OG上面的，解释说由第一个条件可以知道会电脑不会写作的人有20 percent，死活想不出怎么得出来的？

maxwhite

我都是画个表格，比如   

	Comptuter	Not Comptuter	Total
Writing	20%
Not Writing
Total			100%

A说需要电脑技能里，Writing 和 不需要Writing的一样多，则可以知道第2列第3行的百分比也是20%

ximenheng

怎么画怎么得不出那个结论啊。。。

xiangyu4255

韦恩图更清楚些