Question

A geography teacher lists 12 South American countries on the chalkboard. She asks each student in her class to write a report comparing 3 of the countries. How many ways are there to select 3 of the 12 countries

Answer 1

Newton's symbol

$C_{12}^3={12\choose 3}=\frac{12!}{(12-3)!*3!}=\frac{12!}{9!*3!}=\frac{9!*11*\not12^2}{9!*\not3*\not2*1}=11*2=22$

Answer = 22