Given that B = {1, 2, 3, 4}, how many subsets have exactly two elements?

Hello,

If A={1,2,3} then

subsets that have exactly two elementsare

{1,2}, {1,3}, {2,3}

3P2=3*2/2*1=3

If B={1,2,3,4} then

subsets that have exactly two elementsare

{1,2}, {1,3}, {1,4}

{2,3}, {2,4}, {3,4}

4P2=4*3/2*1=6

aPb=a * (a-1) * ... (a-b+1) / b!