How is this the right answer?
In a survey of customer satisfaction, participants are asked to give a score of 1,2,3 or 4 to each of the 6 questions. If participants are instructed not to give the same numerical score to more than 4 questions, how many responses are possible?? I know the answer is 4,020 from the solutions, however I don't understand why. The solution says:
Hence, the number of possible responses is:
Why and how is this the answer? I don't understand where the −4 comes from.