2.已知B总数是240,学science的总数是140,Math 170 求 learn math but not Science? C
A. there are 55 who learn science but not math
B. there are 30 students did not selcet any subject
a)55人学科学==95人两种都学,所以170-95=learn math but not Science
b)30人不学===两者和起来=210,推出两者交集=170+140-210=100,所以learn math but not Science=170-100=70
这样错在那里?请指教