求助各位大虾几道GWD的数学题

死活做不出来了，求助各位帮忙解答，不胜感激

1。if the two-digit integers M and N are positive and have the same digits,but in reverse order.Which of the following CANNOT be the difference of M and N?

a19197 b-99 c43218 d54 e2816

2.A sequence is symmetrically distributed around its arithmetic mean m,and the numbers that fall in the standard deviation d count for 68% of the total numbers in the sequence.What's the percentage of numbers that fall below m+d?

a16% b68% c50% d32% e84%

3.A language class has 155 students.80 of them can speak English.40 of them can speak Russian,60 of them can speak French,There are 50 of them can speak exactly two language,and 10 can speak all three kind languages.What is the number of the student that can speak none of the language?

a7 b20 c15 d45 e25

4.Five red balls and two blue balls are placed in an empty box.One ball a time is to be selected randomly and removed 4 balls from the box.What is the Probability that the first two balls removed will be red and the third ball removed will be blue?

a4/21 b4/7 c5/8 d5/7 e2/5

先谢过各位了 1

1.E 选个不能被9整除的

2.E 1-(1-68%)/2

3.D 155-[10+50+(80+40+60-50*2-10*3)]

4.A (5/7)*(4/6)*(2/5)

OPEN TO DISCUSS

correct!

不过觉的第一题题干有点问题。

3

A∪B∪C=A+B+C-(A∩B+B∩C+C∩A)+A∩B∩C

至少能说一门语言的人数＝ 80 + 40 + 60 - 50 + 10 = 140
一门也不说的人数＝ 155－140 ＝ 15