solution:
A and B have 26 cards which are 7 spades and 19 non spades.
C and D have 6 spades and 20 non spades.
13 cards are chosen randomly from 26 cards (6 spades and 20 non spades)
probability of choosing exactly 4 spades is given by
(6 / 26) × (5/ 25) × (4/24) × (3 / 23) = 3/2990