Chapter 1 - Equations and Inequalities - 1.2 Applications and Modeling with Linear Equations - 1.2 Exercises - Page 103: 38

$\$20,000$ at $3\%$ and $\$80,000$ at $2.75\%$.

Step 1. Assume $x$ amount at $3\%$, thus the amount at $2.75\%$ would be $4x$ Step 2. The total annual interest can be found as $0.03x+0.0275(4x)=2800$ Step 3. Solve the equation: $0.14x=2800$, thus $x=20,000$ Step 4. Conclusion: investments $\$20,000$ at $3\%$ and $\$80,000$ at $2.75\%$.