Bunuel wrote:
There are five different peaches and three different apples. Number of ways they can be divided into two packs of four fruits if each pack must contain at least one apple, is
(A) 95
(B) 65
(C) 60
(D) 30
(E) 27
Solution:Since each pack must contain at least one apple, one pack will contain exactly one apple and the other pack will contain exactly two apples. For the pack with one apple, there are three ways of choosing the apple (and the remaining two apples will go in the other pack).
Assuming the apples are distributed, we need to choose three peaches for the pack containing exactly one apple (and the remaining two peaches will go in the other pack). This choice can be made in 5C3 = 5!/(3!*2!) = (5 x 4)/2 = 10 ways.
Since there are 3 ways to distribute the apples and 10 ways to distribute the peaches after the apples are distributed, there are 3 x 10 = 30 ways to divide the fruit.
Alternate Solution:Let’s calculate the number of ways to divide the fruit into two packs of four without the condition on apples. If the packs were distinguishable, there would have been 8C4 = 8!/(4!*4!) = (8 x 7 x 6 x 5)/(4 x 3 x 2) = 70 ways. However, since the packs are not distinguishable, there are 70/2! = 35 ways to divide the fruit into two packs of four.
Next, notice that the condition “each pack must contain at least one apple” can only fail if all three apples are in one pack. Assuming one of the packs contains all three apples, we have 5 choices for the fourth fruit in that pack (and all the remaining peaches will go in the other pack).
Thus, 5 of the 35 ways to divide the fruit into two packs of four do not meet the criteria and 35 - 5 = 30 of them do.
Answer: D