Răspuns:
28 elevi total
18 f(fotbal)
17b(baschet)
card (AuB)= 28 elevi
card A= 18 f
card B= 17b
card (AnB) = elevi care joacă si f si b
card (AuB)=card A + card B - card (AnB)
28=18+17-card (AnB)
28=35-card (AnB)
card (AnB)=35-28
card (AnB)=7
17-7=10 (elevi care joacă doar baschet)
