GMAT Club Official Solution:A scheduling box contains 12 appointment cards. Each card is for either a morning appointment or an afternoon appointment. If two cards are selected at random without replacement, what is the probability that both selected cards are for afternoon appointments?Let M be the number of morning appointment cards, and let A be the number of afternoon appointment cards.
Since there are 12 cards total:
M + A = 12
The question asks for the probability of selecting two afternoon appointment cards.
(1) The probability of selecting one morning appointment card and one afternoon appointment card is 9/22.
The number of ways to select one morning card and one afternoon card is M * A, and the total number of ways to select 2 cards from 12 is C(12, 2) = 66. So:
MA/C(12, 2) = 9/22
MA/66 = 9/22
MA = 27
Since M + A = 12, the possible values are:
M = 3 and A = 9
or
M = 9 and A = 3
These give different probabilities of selecting two afternoon appointment cards.
Not sufficient.
(2) The probability of selecting two morning appointment cards is 6/11.
So:
M/12 * (M - 1)/11 = 6/11
M(M - 1)/132 = 6/11
M(M - 1) = 72
M = 9
So:
A = 12 - 9 = 3
Therefore, the probability of selecting two afternoon appointment cards can be determined.
Sufficient.
Answer: B.