Question 552395
P(n,r) = (n!)/((n-r)!)


P(21,2) = (21!)/((21-2)!)


P(21,2) = (21!)/(19!)


P(21,2) = (21*20*19!)/(19!)


P(21,2) = 21*20


P(21,2) = 420



So there are 420 different ways.