### Comment:
Rule of 72 'bro'

### REPLY:
I'm aware of what that is, but he has access to a computer and it would take like 10 seconds to do it with the calculator built into google search, especially if he's already taking the time to write out a comment, which was my point. We know n = (1 + r)^t, so just solve for t by doing ln(n)/ln(1 + r). 
  
 ln(100/.42)/ln(1.2) takes several seconds to type and if you do this often enough you have it memorized anyway and don't need to figure it out.

### END.