$100,000 invested at 10% annual return compounded monthly grows to $732,807 in 20 years — $632,807 of that is compound interest. Adding $200 monthly contributions would grow the total to $884,681.

Key Numbers

• •Starting amount: $100,000
• •Value after 20 years at 10%: $732,807
• •Interest earned: $632,807
• •With $200/month added: $884,681
• •At 10%, money doubles roughly every 7.2 years (rule of 72)