$100,000 invested at 10% annual return compounded monthly grows to $270,704 in 10 years — $170,704 of that is compound interest. Adding $200 monthly contributions would grow the total to $311,673.

Key Numbers

• •Starting amount: $100,000
• •Value after 10 years at 10%: $270,704
• •Interest earned: $170,704
• •With $200/month added: $311,673
• •At 10%, money doubles roughly every 7.2 years (rule of 72)