List of top Mathematics Questions on Combinations asked in BITSAT

The total number of $4$-digit numbers in which the digits are in descending order, is

A box contains two white balls, three black balls and four red balls. In how many ways can three balls be drawn from the box if at least one black ball is to be included in the draw?

Total number of books is $2n + 1$. One is allowed to select a minimum of the one book and a maximum of $n$ books. If total number of selections if $63$, then value of $n$ is :