$5,000 invested at 10% annual return compounded monthly grows to $36,640 in 20 years — $31,640 of that is compound interest. Adding $200 monthly contributions would grow the total to $188,514.

Key Numbers

• •Starting amount: $5,000
• •Value after 20 years at 10%: $36,640
• •Interest earned: $31,640
• •With $200/month added: $188,514
• •At 10%, money doubles roughly every 7.2 years (rule of 72)