1. **State the problem:** Find the final amount when $1200 is invested at 5% simple interest for 42 months. 2. **Formula for simple interest:** $$ A = P + I $$ where $A$ is the final amount, $P$ is the principal, and $I$ is the interest. 3. **Interest formula:** $$ I = P \times r \times t $$ where $r$ is the annual interest rate (in decimal), and $t$ is the time in years. 4. **Convert time to years:** $$ t = \frac{42}{12} = 3.5 \text{ years} $$ 5. **Calculate interest:** $$ I = 1200 \times 0.05 \times 3.5 = 1200 \times 0.175 = 210 $$ 6. **Calculate final amount:** $$ A = 1200 + 210 = 1410 $$ **Final answer:** The final amount after 42 months is $1410.