[求助]紧急求助，NN帮忙

想请教一下牛牛这一道，是GWD9里的Q6:

Sue’s monthly earnings consist of a monthly salary and a 4 percent commission on the portion of her monthly sales that is in excess of $2,000.  If Sue’s monthly salary was the same in July as in August, how much greater were her sales in July than in August?（答案是A）

(1)     Sue’s monthly earnings were $3,620 in July and $3,580 in August.

(2)     Sue’s monthly salary was $3,500 in July and in August.

我的看法是，如果这个人的sales没有超过2000,就只能拿到那个salary

(1)3620=salary+4%(X-2000）

     3580=salary+4%（Y-2000)求出X，Y的差

但是如果八月的3580=salary呢？（就是我所说的sales没有超过2000的情形）可以用第一个式子求出X为3000，但是八月的sales就自由了啊？

紧急求助！