That is, the arrangement of five out of seven numbers: A7 takes 5=7*6*5*4*3=2520.

If in a certain order (whether from small to large, from large to small, or in a specified order)

Then: (A7 takes 5) divided by (A5 takes 5)

Namely: 7 * 6 * 5 * 4 * 3/1* 2 * 3 * 4 * 5 = 21.

This is a typical topic of permutation and combination:

A7 Choose 5=7*6*5*4*3=2520 species: this is an arrangement problem.

7 * 6 * 5 * 4 * 3/1* 2 * 3 * 4 * 5 = 21:This is a combinatorial problem.