Answer
$[49\%,99\%)$
Work Step by Step
Step 1. Assume the fifth exam has a score of $x (\%)$. Then the average is given by $\frac{x+95+79+91+86}{5}=\frac{x+351}{5}$
Step 2. To get a B, we need $80\leq \frac{x+351}{5}\lt 90$ or $400\leq x+351\lt 450$ and $49\leq x\lt 99$. Thus the score needs to be in the interval of $[49\%,99\%)$
