GWD一道表格集合问题求助,谢谢

每次一碰到这种集合问题我就晕了

Q20: 

The table above shows the results of a survey of 100 voters
each responded “favorable” or “unfavorable” or “not sure” when asked about
their impressions of candidate M and of candidate N.  What was the number of voters who responded
“favorable” for both candidates?

(1)   The number of voters who
did not respond “favorable” for either candidate was 40.

(2)     The
number of voters who responded “unfavorable” for both candidates was 10.

是选A, 3楼那位大哥,来解释一下吧,谢谢你!

我忘了把答案先贴上来...什么思路呢,这种题碰到好几次,遇一次错一次...

Assume that "x" was the number of voters who responded “favorable” for both candidates.

(1)=> (40-x) + (30-x) + x + 40 = 100 => x=10