GWD 03-31

[briandragon]

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(個位數) 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

請問~~為何III是對的呢~~?不懂

[cocaine]

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.

X: a 7 b
Y: c 5 d

X+Y 四位數. 所以知道a+c+1> 10--> a+c>9

I. 不一定
II. 7+5=12 正確
III. c>5. 因為 X<Y, 且a+c>9 ,所以 a>4, c>5這樣才能進位成4位數.
III正確

ans:II and III

[briandragon]

感恩