A salesperson received a 6-percent commission on the amount of total sales up to and including $10,000, and an r-percent commission on the amount of total sales over $10,000. If the salesperson received a total commission of $920 on total sales of $14,000, what was the value of r?
3.2
4.3
6.6
8.0
9.2
on the amount of sales over (注意这个over,说明只考虑超过这个数额的部分)
920=0.06*10000+r%*4000
r=8