$20,000 invested at 10% annual return compounded monthly grows to $396,748 in 30 years — $376,748 of that is compound interest. Adding $200 monthly contributions would grow the total to $848,846.

Key Numbers

• •Starting amount: $20,000
• •Value after 30 years at 10%: $396,748
• •Interest earned: $376,748
• •With $200/month added: $848,846
• •At 10%, money doubles roughly every 7.2 years (rule of 72)