Let
x ------> score on the fourth test
we have that
[tex]\frac{76+76.5+82.75+x}{4}\ge80[/tex]Remember that
at least, means -----> "greater than or equal to"
so
solve for x
[tex]76+76.5+82.75+x\ge80\cdot4[/tex][tex]235.25+x\ge80\cdot4[/tex][tex]\begin{gathered} x\ge320-235.25 \\ x\ge84.75 \end{gathered}[/tex]therefore