A={ x aparține N | x<2001}
x<2001 => x={0,1,2,3,...,2000}
A={0,1,2,3,...,2000}
Card A=2001 deoarece 2000-0+1
B={ x aparține N* | x ≤ 1957}
x ≤ 1957 => x ={1,2,3,...,1957}
B={1,2,3,...,1957}
Card B = 1957
C={ x aparține N | 4 ≤ x < 2002}
4 ≤ x < 2002 => x = { 4,5,6,7,...,2001} = C
Card C = 2000