求PREP-DS-2-155 解

155.
                15795-!-item-!-187;#058&010817

If 500 is the multiple of 100 that is closest to x and 400 is the multiple of 100 that is closest to y, which multiple of 100 is closest to x + y ?

(1) x < 500

(2) y < 400

求思路,谢谢

距离x最近的100的倍数是500，则 451<x<549;距离y最近的100的倍数为400，则351<y<451

1) 推出451<x<500  ，则802<x+y<951 ，满足最接近x+y的100的倍数有800或900

2）同上

结合1）2）推出802<x+y<900  ，之后同上

非常感谢,

速度真快啊~~~~