[求助]蓝皮书数学DS题第206题

206.                          favorable     unfavorable    not sure

             candidate M      40                   20                 40

             candidate N      30                   35                   35

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 candidates was 40.

(2)the number of voters who responded unfavorable for both candidates was 10.

参考答案是A，我没有搞明白。求助大虾。