How much more interest will Maria receive if she invests $1,000 for one year at x percent annual interest, compounded semiannually, than if she invests $1,000 for one year at x percent annual interest, compounded annually?
Last month a phone company charged $0.07 per minute for the first 300 minutes of calling time and $0.05 per minute for each minute of calling time in excess of the first 300 minutes. For how many minutes of calling time was Bob charged by the phone company last month?
(1) Last month the phone company charged Bob $21.00 for his calling time that was charged at the rate of $0.07 per minute.
(2) Last month the phone company charged Bob $40.00 for his calling time that was charged at the rate of $0.05 per minute.
还有这道为什么(1) 是不对的啊