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 withdrawals 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 $2,600, 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 $2,625.
(2)Last year the closing balance of Carl's account for June was less than $2,675.
要得到这个答案，就必须知道两点：N的确定值和账户中原来就有的金额A.
这两点确定了则每个月的余额都能计算出来，那么范围就出来了。所以这道题的实质是要确定N与A为多少。
由已知条件得：
若N>=5, 5*120+A=2600
若N<5, 120N+A-50(5-N)=2600
