$20,000 invested at 7% annual return compounded monthly grows to $162,330 in 30 years — $142,330 of that is compound interest. Adding $200 monthly contributions would grow the total to $406,324.

Key Numbers

• •Starting amount: $20,000
• •Value after 30 years at 7%: $162,330
• •Interest earned: $142,330
• •With $200/month added: $406,324
• •At 7%, money doubles roughly every 10.3 years (rule of 72)