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?
(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.
key：A
mine：c
3580=Salary August+4%(Sales August-2000) b
会不会有Sales August<2000的情况呢
这样那两个式子就不成立 | 
