The sum of the units digits of all numbers formed using the digits $3, 4, 5, 6$ without repetition is......

${ }^{20}P_5 - { }^{19}P_5 = $

If the letters of the word $COCHIN$ are rearranged and all permutations are arranged in alphabetical order as in an English dictionary,how many words appear before the word $COCHIN$?

How many $4$-digit numbers are there with no digit repeated?

If four-digit numbers are formed by using the digits $1, 2, 3, 4, 5, 6, 7$ without repetition,then how many of these numbers are exactly divisible by $25$?

$6$ boys and $5$ girls sit in a line such that $(I)$ no two girls sit together $(II)$ all the girls sit together. If $p$ is the number of arrangements in case $(I)$ and $q$ is the number of arrangements in case $(II)$,then $p/q =$