# Chapter 3 - Solving Inequalities - 3-6 Compound Inequalities - Practice and Problem-Solving Exercises - Page 205: 42

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 $6.3\leq p\leq8.1$

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