英文数学题 要求列方程解答

题目

英文数学题 要求列方程解答
An investor received $1400 interest per annum from a sum of money, with part of it invested at 10% and the remainder at 7% simple interest.This investor found that if she interchanged the amounts she had invested she could increase by $90 per annum.Calculate the total amount invested

答案

Let the amounts invested in 10% and 7% x and y dollars respectively:
x*10% + y*7% = 1400
x*7% + y*10% = 1400 + 90
y = 10000
x = 7000
Total is $17000