数学prep， 读不懂

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

读不懂什么意思，请教

如果500是最接近X的100的倍数，400是最接近Y的100的倍数，那么最接近X+Y的100的倍数是多少？