Simple interest uses $I = prt$. Here $I = 1200 \times 0.035 \times 4 = 168$ dollars. Compound interest (for comparison) uses $A = p(1+r)^t$: $A = 1200(1.035)^4 \approx 1377.03$, so compound interest earned $\approx 177.03$ dollars. Simple interest is on principal only, while compound interest earns interest on interest, so compound grows faster over time. Knowing this helps you choose savings accounts that can grow your money more effectively.

Simple interest: $I = prt = 2000 \times 0.04 \times 5 = 400$ dollars, so final $= 2400$. Compound interest: $A = p(1+r)^t = 2000(1.04)^5 \approx 2433.31$, so interest $\approx 433.31$ dollars. Difference $\approx 433.31 - 400 = 33.31$ dollars in favor of compound. Simple interest is on principal only; compound earns interest on interest, so it grows faster—useful when comparing savings and investment accounts.

Bank A (simple): $I = 1500 \times 0.03 \times 6 = 270$, final $= 1770$ dollars. Bank B (compound): $A = 1500(1.028)^6 \approx 1770.31$ dollars. Compound is slightly higher by about $0.31$ dollars because compound interest adds interest on interest. Even small differences can matter over time when choosing accounts.

Simple: $I = 800 \times 0.05 \times 3 = 120$ dollars. Compound: interest $= 800(1.05)^3 - 800 = 800(1.157625 - 1) = 126.10$ dollars. Difference $= 126.10 - 120 = 6.10$ dollars. Simple interest is on principal only; compound includes interest on interest, which is why it grows a bit faster even over 3 years.

Bank A (simple): $I = 3000 \times 0.05 \times 2 = 300$ dollars. Bank B (compound): interest $= 3000[(1.045)^2 - 1] \approx 3000(1.092025 - 1) = 276.08$ dollars. Difference $= 300 - 276.08 = 23.92$ dollars in favor of simple. Simple interest can win over short periods if its rate is higher, while compound generally wins as time increases because it earns interest on interest.