Question

The water acidity in a swimming pool is considered normal when the average pH level of three daily measurements is between 7.2 and 7.8. If first two pH readings are 7.48 and 7.82, find the range of pH value for the third day so that the acidity level is within the normal range.

Answer 1

Let x be the third reading. 

The average must satisfy 7.2 ≤ `(7.48 + 7.82 + x)/3 ≤ 7.8`.

`7.2 ≤ (15.30 + x)/3 ≤ 7.8`

21.6 ≤ 15.30 + x ≤ 23.4

6.30 ≤ x ≤ 8.10

So the third-day pH must be between 6.30 and 8.10 (inclusive) for the average to lie between 7.2 and 7.8.