4. April took out a $\mathrm{\$}600$ loan from the bank. At the end of 5 years, she pays back the principal, plus $\mathrm{\$}60$ 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: $60=600\ast R\ast 5$.

Step 4 ：Solving for R, we get: $R=\frac{60}{600\ast 5}$.

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: $I=PRT$, $I=600\ast 0.02\ast 5$, $I=\$60$.

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

Step 9 ：$\boxed{{\displaystyle R=0.02}}$ or $\boxed{{\displaystyle 2\mathrm{\%}}}$ when expressed as a percentage.