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≤$\frac{7.4+7.9+p}{3}$≤7.8⟶ multiply each part by 3
7.2×3≤7.4+7.9+p≤7.8×3⟶ multiply
21.6≤15.3+p≤23.4⟶ subtract 15.3 from each part
21.6−15.3≤15.3+p−15.3≤23.4−15.3⟶ subtract
6.3≤p≤8.1
