5.三个考官考30学生，A认为15名过，B认为17名过，C认为18名过。只有三人都过者才叫过，问至少有几人过了
我认为是1，AB交集至少2个，AC交集至少3，BC交集至少1.，再交集，就是1。
确认:0
Robert的解:
这个题是不是题目不全？但上面的解法肯定有不妥之处。先看公式:
|A∩B∩C|=|A∪B∪C|-|A|-|B|-|C|+|A∩B|+|A∩C|+|B∩C|
由本题可知:|A∩B∩C|=30-15-17-18+|A∩B|+|A∩C|+|B∩C|>=0
这道题我有些意见，我认为答案是2，可以用维恩图画，此外也可以列式，15+17+18-30=20，20-max(15,17,18)=20-18=2。一定有2个cover不到的。
此题在CXD的书中出现过，数字不同，CXD中是24,15,7, 24+15+7=46, 46-30=16, 16
I don't think using these formula is a good way.
try this
let's think how could we calulate this question?
2考官考30学生，A认为15名过，B认为17名过, then AT LEAST how many students pass A & B,
then almost every one could get the answer it is 17+15-30=2
then lets return to this question,
三个考官考30学生，A认为15名过，B认为17名过，C认为18名过。只有三人都过者才叫过，问至少有几人过了
Compare these two question, we still could use similiar way to solve it
thinking: 17+15+ 18-30*2 < 0 so at least 0.
then even if it is 4(5,6,7)个考官考学生, we still could get it easily
A pass+ Bpass + C pass+.....-30*(n-1)=
it is the number for AT LEAST
1.some products total costs $54.64 exclusive tax.The tax is $0.79 which is 5% rate to the taxable items. The untaxed costs
My answer: 39.63确认:39.63
解:令untaxed costs b $,需交的为a,则有 a+b=x,5%a=0.79,x-0.79=54.64,所以(54.64+0.79)-0.79/5%=39.63，这样比较好理解了吧。
39.63Robert的答案，我认为是正确的。
