Answer
See below.
Work Step by Step
Let $x$ be the amount of $3$ ounce tea used, then the amount of $2.75$ ounce tea used is $80-x$. Then the equation of the mix is: $3x+2.75(80-x)=2.9\cdot80\\3x+220-2.75x=232\\0.25x=12\\x=48$
Thus he used $48$ ounces of the $3$ dollar tea and $32$ ounces of the $2.75$ dollar tea.
