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GMAT/Problem Solving - Girls & Boys
Girls & Boys
Ms. Barton has four children. You are told correctly that she has at least two girls but you are not told which two of her four children are those girls. What is the probability that she also has two boys? (Assume that the probability of having a boy is the same as the probability of having a girl.)
(A) 1/4
(B) 3/8
(C) 5/11
(D) 1/2
(E) 6/11
ОТВЕТЫ И КОММЕНТАРИИ
Решение
Since each of the 4 children can be either a boy or a girl, there are possible ways that the children might be born, as listed below:
Since we are told that there are at least 2 girls, we can eliminate 5 possibilities--the one possibility in which all of the children are boys (the first row) and the four possibilities in which only one of the children is a girl (the second row).
That leaves 11 possibilities (the third, fourth, and fifth row) of which only 6 are comprised of two boys and two girls (the third row). Thus, the probability that Ms. Barton also has 2 boys is 6/11 and the correct answer is E.
In a 4 person race, medals are awarded to the fastest 3 runners. The
first-place runner receives a gold medal, the second-place runner receives a
silver medal, and the third-place runner receives a bronze medal. In the event
of a tie, the tied runners receive the same color medal. (For example, if there
is a two-way tie for first-place, the top two runners receive gold medals, the
next-fastest runner receives a silver medal, and no bronze medal is awarded).
Assuming that exactly three medals are awarded, and that the three medal
winners stand together with their medals to form a victory circle, how many
different victory circles are possible?
Вверх 
