The watering acidity in a pool is considered normal when the average

Let x be the third pH value.

Now, it is given that the average pH reading of three daily measurements is between 8.2 and 8.5

Also, the first two pH readings are 8.48 and 8.35

Therefore,

$8.2<\frac{8.48+8.35+x}{3}<8.5$

Multiplying throughout by 3 in the above equation

$3(8.2)<\left(\frac{8.48+8.35+x}{3}\right)(3)<3(8.5)$

$24.6<8.48+8.35+x<25.5$

$24.6<16.83+x<25.5$

Subtracting throughout by 16.83 in above equation

$24.6-16.83<16.83+x-16.83<25.5-16.83$

$7.77

Therefore

The value of third pH reading ranges from 7.77 to 8.67