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# On the occasion of Diwali each student of a class sends greeting cards to others. If there are 20 students in a class, then how many greeting cards were exchanged?

$\begin{array}{1 1} ^{20}C_2 \\ 2.^{20}C_2 \\ ^{20}P_2 \\2.^{20}P_2 \end{array}$

Each student will send 19 greetings.
There are 20 students.
$\therefore$ The no. of greetings exchanged = $20\times 19=2.^{20}C_2$