How much did he invest in each account?

Question

Last year, boris had $30,000 to invest. He invested some of it in an account that paid 7% interest per year, and he invested the rest in an account that paid 5% interest per year. After one year, he received a total of $1820 in interest.

Answer 1

Let's see. He invests say X dollars at 7% and $30,000 - X at 5%, and gets $1820 in interest.

Therefore

0.07 X + 0.05 (30,000 - X) = 1820.

One equation, one unknown - solve for X.