Of the companies surveyed about the skills they required in prospective employees, 20 percent required both computer skills and writing skills. What percent of the companies surveyed required neither computer 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.

The answer key states in #1, you can get the % of companies reqireing computer skills but not requiring writing skills. I understand that. What I don't understand is the number, which is 20%. Can anyone answer this?