求解OG13DS的第143题，大神们请帮帮忙！

krusher

如题，求好心人详解！

Beginning in January of last year, Carl made deposits of $120 into his account on the 15th of each month for several consecutive months and then made withdrawls of $50 from the account on the 15th of each of the remaining months of last year. There were no other transactions in the account last year. If the closing balance of Carl's account for May of last year was $2600, what was the range of the monthly closing balances of Carl's account last year?

(1)last year the closing balance of Carl's account for April was less than $2625

(2)last year the closing balance of Carl's account for June was less than $2675

honeybee316

这题的意思是这个人跑上来每个月都会存120，但是到了某个月开始不存了，开始提50。
也就是说，要么下个月比前个月高120，要么就是少50。
现在给出的条件是5月份时2600块。（PS：这个人在年头的时候账户里有余额，所以总额会有2600）
1）4月底<2625，5月份2600
如果5月份没存的话，也就是说5月份就到不了2600（2625-50<2600)。所以5月份存了。
2）6月底<2675，5月份2600
如果6月份存的话，那应该是2600+120=2720，但现在<2675，说明6月份没存，开始提50了。
结合1）2）可以得知1-5月是存款，6-12月是提款。

krusher

感谢

louisehu

其实我想问题目的那个range是什么啊？ 本题要解答的到底是啥啊。。

julia_juer

range应该是这么多月份中，closing balance最大值与最小值只差吧