Number of ways a sub committee can be formed Twelve doctors are to be selected by hospital staff to sit on a six-person committee. Within that committee, an additional subcommittee of three doctors will be formed. How many unique sub-committees are possible?...... Please let me know

Since we want the number of unique subcommittees we know that there are ${12\choose 3}=220$ unique subcommittees. The reason we know this is because in total there are $12$ doctors and a subcommittee has $3$ doctors in it. We want all of the unordered selections of $3$ of the $12$ doctors.