$10,000 invested at 10% annual return compounded monthly grows to $73,281 in 20 years — $63,281 of that is compound interest. Adding $200 monthly contributions would grow the total to $225,155.

Key Numbers

• •Starting amount: $10,000
• •Value after 20 years at 10%: $73,281
• •Interest earned: $63,281
• •With $200/month added: $225,155
• •At 10%, money doubles roughly every 7.2 years (rule of 72)