Let’s say you’re 50 years-old and you have an extra $900 a month to work with. Should you use that money to pay down your mortgage or contribute it to your 401(k)? Assume you just refinanced your mortgage for $450,000 over 15 years at 3.5 percent interest— so you’re paying $3,217 a month. Also, assume you have $500,000 in your 401(k) account and you’re putting away $10,000 annually, which is growing at 7 percent.