Answer: A customer can add 2/9 of milk to the cup.
Explanation:
Hi, to answer this we have to subtract the amount of coffee in the cup ( 7/9 of a pint) to the total capacity of the cup (1).
Mathematically speaking
1- 7/9 = 2/9
2/9 of a cup is the capacity left for adding milk.
A customer can add 2/9 of milk to the cup.
Feel free to ask for more if needed or if you did not understand something.