$20,000 invested at 10% annual return compounded monthly grows to $54,141 in 10 years — $34,141 of that is compound interest. Adding $200 monthly contributions would grow the total to $95,110.

Key Numbers

• •Starting amount: $20,000
• •Value after 10 years at 10%: $54,141
• •Interest earned: $34,141
• •With $200/month added: $95,110
• •At 10%, money doubles roughly every 7.2 years (rule of 72)