Answer:
the third reading should be between 6.3 and 8.1 for the acidity to be normal
Step-by-step explanation:
if the average m should be between 7.2 and 7.8, then
7.2 ≤ m ≤ 7.8
if m = (r₁+r₂+r₃)/3 , where r represents the readings , then:
(r₁+r₂+r₃)/3 ≤ 7.8
but the fist reading r₁=7.4 and the second one r₂=7.9 , then
(7.4+7.9+r₃)/3 ≤ 7.8
7.4+7.9+r₃≤ 3*7.8
r₃≤ 3*7.8 - 7.4 - 7.9 = 8.1
then
r₃≤ 8.1
also 7.2 ≤ (r₁+r₂+r₃)/3 , then
7.2 ≤ (7.4+7.9+r₃)/3
3* 7.2 ≤ 7.4+7.9+r₃
3* 7.2 - 7.4- 7.9 ≤ r₃
6.3≤ r₃ → r₃ ≥6.3
if r₃ ≥6.3 and r₃≤ 8.1 , then
6.3 ≤r₃≤ 8.1
the third reading should be between 6.3 and 8.1 for the acidity to be normal
