[markcampbell]

He/she would only have to pay 3.28 times his normal payment in order to pay it off in 1/5th of the time. It's paid off more quickly because by paying more, you reduce the amount of interest that accumulates.

On a $100k 20 year loan at at 5.7% (I know, example), interest paid would be around $67k (so total $167k). By paying 3.28 times more per month ($2300 per month versus the $700 normal rate), the interest paid ends up being only around $12k.