[求助]请教一道数学题

fuhongler

Q19:

A total of $60,000 was invested for one year.  But of this amount earned simple annual interest at the rate of x percent per year, and the rest earned simple annual interest at the rate of y percent per year.  If the total interest earned by the $60,000 for that year was $4,080, what is the value of x?

(1)     x = (3/4) y

(2)   The ratio of the amount that earned interest at the rate of x percent per year to the amount that earned interest at the rate of y percent per year was 3 to 2.

答案是C

hugewaters

根据题意，可列出式子 x%*a+y%*(60000-a)=4080

由条件（1）可以得出 x,y的关系 三个未知数即可化为二个

有条件 (2) 可以得出 a/(60000-a)=3/2 即可求出a

这样一个方程 一个未知数 就可以求解咯~~~

呵呵~~仅供lz参考啦~~

ballparks

如果只有条件2的华，只能知道a，但是x，y不知，仍旧是一个方程两个未知数。所以不可以单独解答出答案，所以综合1，2一起，可以解决。

trifling

结合1,2可以求出第一部分资金是40000,第二部分20000

再40000X+20000Y=4080,X=3/4Y

得到X=6.12%