M JJ 78

yinge1019

Q31:

The numbers x and y are three-digit positive integers, and x + y is a four-digit integer.  The tens digit of x equals 7 and the tens digit of y equals 5.  If x < y, which of the following must be true?

 

 

  I.  The units digit of x + y is greater than the units digit of either x or y.

        II.  The tens digit of x + y equals 2.

       III.  The hundreds digit of y is at least 5.

 

 

A.   II only

B.   III only

C.   I and II

D.   I and III

E.    II and III

  Answer: B

rainsun

B

X < Y,并且 X 十位大于Y，
所以X百位 一定小于Y，
又想 百位相加>9, 所以 Y 的百位要>5, 因为不大于5，就不能进位到千位.

I and II are not necessarily true for example,
x = 476, and y=555.

[此贴子已经被作者于2008-11-19 5:22:52编辑过]