We want to know how many brownmies did Fred and his family eat together.
We will call to the total of the brownies by 1. On this case, after Fred ate 1/3 of the brownies, he will have:

This means that he has left 2/3 of the brownies. After his family ate 1/6 of the brownies:

This means they will have left 1/2 of the tray of brownies, and that they ate half of it.