Answer
Average $=\dfrac{s+160}{3}$
Work Step by Step
Let $s$ = the third test score
The average of three scores can be found by adding the three scores together then dividing the sum by three.
This means that the average of the three test scores is:
$=\dfrac{78+82+s}{3}
\\=\dfrac{160+s}{3}
\\=\dfrac{s+160}{3}$
