$3, 7$ different positive integers arranged in ascending order form an arithmetic sequence. It is known that the average of the first three numbers is 20, and the average of the last three numbers is 24. Then, the average of the middle three numbers is $\qquad$ -
【Analysis】The average of the first three numbers is 20, which means the 2nd number among the 7 numbers is 20. The average of the last three numbers is 24, which means the 6th number among the 7 numbers is 24. Therefore, the 4th number among the 7 numbers is $(20+24) \div 2=22$, which also means the average of the middle three numbers is 22.