Question Number 124376 by bemath last updated on 02/Dec/20
Answered by liberty last updated on 02/Dec/20
$${n}\left({A}\right)=\:{C}_{\mathrm{1}} ^{\:\mathrm{10}} \:=\:\mathrm{10} \\ $$$${n}\left({S}\right)\:=\:{C}\:_{\mathrm{2}} ^{\mathrm{20}} \:=\:\frac{\mathrm{20}×\mathrm{19}}{\mathrm{2}×\mathrm{1}}\:=\:\mathrm{190}\: \\ $$$${p}\left({A}\right)\:=\:\frac{\mathrm{10}}{\mathrm{190}}\:=\:\frac{\mathrm{1}}{\mathrm{19}}. \\ $$