[問題]GWD4-31

[haijung]

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


答案(B), 我選(E)
我想請位各位高手, 為什麼II不對? X的十位數是7, Y的十位數是5, X+Y的十位數為什麼不是2呢?

謝謝~~

[65kevin]

因為 X 和 Y 的個位數並不知道是多少, 有可能會進位, 影響到兩數相加的十位數

[haijung]

對喔對喔~~ 多謝指點~~ 感恩~~