(b) \( \$ 3000 \) simple interest is paid on an investment of \( \$ 15000 \) after 4 years. Find the interest rate \( (R) \).

To find the interest rate \( R \), you can use the simple interest formula: \[ I = P \times R \times T \] where \( I \) is the interest paid, \( P \) is the principal amount, \( R \) is the rate as a decimal, and \( T \) is the time in years. In this case, you know that \( I = 3000 \), \( P = 15000 \), and \( T = 4 \). Rearranging the formula gives: \[ R = \frac{I}{P \times T} = \frac{3000}{15000 \times 4} = \frac{3000}{60000} = 0.05 \] Converting it to a percentage means multiplying by 100, so \( R = 5\% \). So, the interest rate is \( 5\% \).