Explanation:
According to the given information,
There are 4 lines with 7 customer each, so to calculate the "total number of customers",
(one line with 7 customer so 4 line = ?)
4 * 7 =28
Total customers = 28
There are 5 customer who are left. So the "number of customers" whose service is done can be calculated as,
28-5 = 23
Customer serviced = 23
Left out customer = 5
