In this question, there are numerous important information's that needed to be taken note of. It is already given that 1/12 is the commission received by the salesman on the sales price of a car.
Then
Sales commission of the salesman on the car = (1/12) * 21999
= 21999/12
= 1833.25 dollars
So the salesman receives a commission of $1833.25 on the sales of the car that was priced at $21999. I hope you have understood the method by which the problem has been solved.