The topic is not clearly written, so it's better to write it as follows. The economic mathematics team will help you answer, please comment in time. thank you

135…(2n- 1)246…(2n)

From front to back: 3 and below 2 constitute the reverse order,1; 5 and below 24 constitute the reverse order, with 2; ..., (2n- 1) and the following 246...(2n-2) all constitute the reverse order, with n-1; So the inverse number is1+2+…+(n-1) = n (n-1)/2.

246…(2n) 135…(2n- 1)

From beginning to end: 2 and below 1 constitute the reverse order, with1; 4 and below 13 constitute the reverse order, and there are two; ..., (2n) and below 135...(2n- 1) all constitute reverse order, and there are n; So the inverse number is 1+2+…+n=n(n+ 1)/2.