请教三题陈向东数学

第一章

85. If S is an infinite set of real numbers,is there a number in S that is less than every other number in S?

(1)Every number in S is an integer

(2)Every number in S is positive

答案说C，但是我觉得应该是D。每个条件都能推出there is not a number in S that is less than ……，不是推出否定的结果也算对吗？

99. In Township K,1/5 of the housing units are equipped with CT（有线电视）,.If 1/10 of the housing units,including 1/3 of those that are equipped with CT,are equipped with VR（录像）,what fraction of the housing units have neither CT nor VR?

A.23/30  B 11/15   C.7/10  D.1/6  E.2/15

答案是A,我觉得是B。答案解释，配有有线电视的住房的1/3配有录像，说明有1/15的住房两个都有配。可是我根据题意，觉得他说反了，应该是1/10配有录像，这里面又有1/3配有有线电视（不知我理解的对否），所以根据这个两个都配的应该是1/10*1/3=1/30.

100.Each of the integer from 0 to 9,inclusive,is written on a seprate slip of blank paper and the ten slips are dropped into a hat. If the slips are then drawn one at a time without replacement, how many must be drawn to ensure that the numbers on two of the drawn will have a sum of 10?

这题不会

请高人指教，谢谢！

85. (1)只说明该集合里都是整数，那就包括正数和负数，负数是无穷小的。所以不充分

    （2）只说明是正数，就包括小数和整数，正小数也是可以无穷小的，也不充分

     1+2 正整数有最小值1

99。including 1/3 of those that are equipped with CT 意思是包括1/3安装了CT的房间，不是1/10中的1/3 。

100。书后面又提到的抽屉原理，0-9的10个数字中 （1， 9）， （2，8），（3，7），（4，6） 四对相加等于10， 要保证抽出来的数字相加等于10 ，要抽4+2+1=7次， 4=四组中每组抽出一个， 2=5 和0 ， 1=从四组中再抽任一个。