**Hint** : Let $n$ be the number of sales Matt has made, $T$ the total commission Matt has earned, including the most recent sale. How would you calculate his current average commission (if you didn't know it already) from these values? How would you calculate the average commission prior to his most recent sale? From these calculations, you will have two linear equations in the two variables $n$ and $T$, from which you should be able to determine $n$ (and $T$, of course).