Since, the largest digit is in the middle, the middle digit is greater than or equal to 5 , the number of numbers with 5 in the middle
=4P4 (
∵ the four places are to be filled by
1,2,3,4 )
Similarly, the number of numbers with 6 in the middle
=5P4 The number of numbers with 7 in the middle
=6P4 The number of numbers with 8 in the middle
=7P4 The number of numbers with 9 in the middle
=8P4 ∴ Total number of such numbers
=4P4+5P4+6P4+7P4+8P4=rP4