The number of four-digit natural numbers which contain exactly two distinct digits is:

The number of $5$-digit numbers that can be formed using the digits $1, 2, 3, 4, 5, 6$ such that the number must include both $1$ and $2$ is:

The letters of the word $ASSASSIN$ are written down at random in a row. The probability that no two $S$ occur together is

There are $10$ red and $5$ yellow roses of different sizes. If $x$ is the number of garlands that can be formed with all these flowers so that no two yellow roses come together and $y$ is the number of garlands formed with all these flowers so that all the red roses come together,then $\frac{2(x-y)}{10!}=$

The number of integers $n$ such that $100 \leq n \leq 999$ and $n$ contains at most two distinct digits is:

The number of words that do not start and end with vowels,formed using all the letters of the word $'UNIVERSITY'$ such that all vowels are in alphabetical order,is