The compound inequality which is used to model the normal range of pH values of water is:
x>6.5 and x<8.5 ( 6.5<x<8.5)
It is given that:
Normal drinking water needs to have a pH of less than 8.5 and greater than 6.5.
This means that the pH level of the water will lie in the range in between 6.5 and 8.5 excluding them.
Hence, if the pH level is less than or equal to 6.5 or greater than or equal to 8.5 then it could not be used for drinking purpose.
If x represent the pH of the normal drinking water then:
6.5 < x < 8.5
i.e. x>6.5 and x<8.5
