Take three even numbers and four odd numbers from a number and ask: ① How many can be formed?

A single number has five even numbers and five odd numbers. The seven digits that can be formed by repeatedly taking three even numbers and four odd numbers are c (5,3) c (5,4) a (7,7)-c (4,2) c (5,4) a (6,6) = 252000-21600 = 230400.

In the formula, c (5,3) c (5,4) a (7,7) is 3 of 5 even numbers, 4 of 5 odd numbers and the arrangement of ***7 numbers, which is the first arrangement of 7 numbers including 0. C (4,2) C (5,4) A (6,6) is the arrangement of 4 even numbers except 0, 4 odd numbers and ***6 numbers, and it is the arrangement of the remaining 6 numbers when 0 is in the first place.