Answer
pH readings between 6.3 and 8.1 inclusive would make the average pH normal.
Work Step by Step
Let p be the pH of the third reading. If the average is between 7.2 and 7.8 inclusive, this is represented by a compound inequality using "and."
$7.2\leq\frac{7.4+7.9+p}{3}\leq7.8\longrightarrow$ multiply each part by 3
$7.2\times3\leq{7.4+7.9+p}{}\leq7.8\times3\longrightarrow$ multiply
$21.6\leq15.3+p\leq23.4\longrightarrow$ subtract 15.3 from each part
$21.6-15.3\leq15.3+p-15.3\leq23.4-15.3\longrightarrow$ subtract
$6.3\leq p\leq8.1$
