The water acidity in a pool is considered normal when the average pH reading of three daily measurements is between 7.2 and 7.8.

Let x be the third  pH value.

Then, $7.2<\frac{7.48+7.85+x}{3}<7.8$

$\Rightarrow 7.2<\frac{15.33+x}{3}<7.8$

$\Rightarrow 21.6<15.33+x<23.4 \quad$ (Multiplying throughout by 3 )

$\Rightarrow 21.6-15.33<15.33+x-x<23.4-15.33$

$\Rightarrow 6.27

Hence, the range for the $\mathrm{pH}$ value for the third reading must be between $6.27$ and $8.07$