Consider that a retired couple has $420,000 to invest. They chose one relatively safe investment fund that has an annual yield of 6% and another riskier investment that has a 10% annual yield.

Let us find how much they should invest in each fund to earn $30,000 per year.

Denote the fund invested at 6% as x and the fund invested at 10% as y.

Then we have the equation

...... (1)