Given that the student's grades on three tests are 80, 81 and 70
Let the student's grade on the fourth test be X
In order to pass the course, the student must have an average of 80 or greater.
Hence,
[tex]\begin{gathered} \frac{80+81+70+X}{4}\ge80 \\ \frac{231+X}{4}\ge80 \\ 231+X\ge320 \\ X\ge320-231 \\ X\ge89 \end{gathered}[/tex]The score the student must get on the next test to get a B average or better is greater than or equal to 89