<prompt> Human: Family member has 100K to either pay down mortgage, of which 150K is remaining, or put in an index fund. Hes five years into a 15 year refi at 3.5 fixed. Im genuinely torn and wanted to get yalls views. I know the conservative approach is pay down the mortgage, but whats got the biggest expected payoff in ten years, say?EDIT: Thanks for all the advice, everyone. Im going to recommend he put it all in index funds. This money is all play money in a sense, as it was an unexpected inflow, so he can afford to go for the highest expected value moveie if it all drops 10 tomorrow hell be okay. After deductions, with the low real interest rate, it seems optimal to go for the highest return for the life of the mortgage 10 years.  <response>: As others have said, its a risk assessment, guaranteed 3.5 gain vs probable higher gain, yada yada. One thing no one else has mentioned, I assume this person has more than 20 in home equity? If hes paying PMI, get rid of that. If not, Id max retirement accounts for the year or maybe the next few years. 