4. April took out a $\$ 600$ loan from the bank. At the end of 5 years, she pays back the principal, plus $\$ 60$ simple interest. What was the annual interest rate?

Step 1 ：Given in the problem: Principal (P) = $600, Interest (I) = $60, Time (T) = 5 years. We need to find the rate (R).

Step 2 ：The formula for simple interest is \(I = PRT\), where I is the interest, P is the principal, R is the rate, and T is the time.

Step 3 ：Substituting the given values into the formula, we get: \(60 = 600 * R * 5\).

Step 4 ：Solving for R, we get: \(R = \frac{60}{600 * 5}\).

Step 5 ：Solving the above equation, we get \(R = 0.02\).

Step 6 ：So, the annual interest rate is 0.02, or 2% when expressed as a percentage.

Step 7 ：Let's check our answer: If we substitute \(R = 0.02\) back into the formula, we get: \(I = PRT\), \(I = 600 * 0.02 * 5\), \(I = $60\).

Step 8 ：This matches the interest given in the problem, so our answer is correct. The annual interest rate is 2%.

Step 9 ：\(\boxed{R = 0.02}\) or \(\boxed{2\%}\) when expressed as a percentage.