Ira Schwab wins the lottery and decides to take the one lump sum of $500,000 minus taxes.

Question:

Ira Schwab wins the lottery and decides to take the one lump sum of $500,000 minus taxes. Ira receives a check for $300,000 after taxes. Using the rule of 72, determine
a. How long will it take him to get $600,000 if he can earn 3 percent?
b. How long will it take him to get $600,000 if he can earn 6 percent?
c. How long will it take him to get $1,200,000 if he can earn 9 percent?