$20,000 invested at 7% annual return compounded monthly grows to $40,193 in 10 years — $20,193 of that is compound interest. Adding $200 monthly contributions would grow the total to $74,810.

Key Numbers

• •Starting amount: $20,000
• •Value after 10 years at 7%: $40,193
• •Interest earned: $20,193
• •With $200/month added: $74,810
• •At 7%, money doubles roughly every 10.3 years (rule of 72)