$20,000 invested at 10% annual return compounded monthly grows to $146,561 in 20 years — $126,561 of that is compound interest. Adding $200 monthly contributions would grow the total to $298,435.

Key Numbers

• •Starting amount: $20,000
• •Value after 20 years at 10%: $146,561
• •Interest earned: $126,561
• •With $200/month added: $298,435
• •At 10%, money doubles roughly every 7.2 years (rule of 72)