请教4道prep中的DS题

kikisunflower

第一题不懂什么意思，什么叫W B R个球哦？

第四题：
the multiple of 100的意思是，以00结尾的所有数，比如100,200,300...99000...

500 is the multiple of 100 that is closest to x：x=600 or 400
400 is the multiple of 100 is closest to y： y=300 or 500