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nkmungila
Zachary wants to buy four ice­creams ­ one each of four different brands. If each of the four brands serve three different flavors ­ chocolate, vanilla, and strawberry, how many different combinations of ice­creams can he buy, provided that he doesn’t end up buying all ice­creams of the same flavor?

A. 60
B. 78
C. 126
D. 156
E. 256


hi..

four different brands and each having 3 flavours..
total ways to choose 4 with the given restriction that one of each brand is chosen = \(3*3*3*3 = 81\)

ways in which all flavours are same = 1*1*1*1*3= 3, since there are three flavours

so answer = \(81-3=78\)

B

Can you please explain the highlighted portion?
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nkmungila
Zachary wants to buy four ice­creams ­ one each of four different brands. If each of the four brands serve three different flavors ­ chocolate, vanilla, and strawberry, how many different combinations of ice­creams can he buy, provided that he doesn’t end up buying all ice­creams of the same flavor?

A. 60
B. 78
C. 126
D. 156
E. 256


hi..

four different brands and each having 3 flavours..
total ways to choose 4 with the given restriction that one of each brand is chosen = \(3*3*3*3 = 81\)

ways in which all flavours are same = 1*1*1*1*3= 3, since there are three flavours

so answer = \(81-3=78\)

B

Can you please explain the highlighted portion?

Hi,
Your query can be easily clarified using the slot method, which I have explained below.

The number of different combinations of ice­creams that he can buy, provided that he doesn’t end up buying all ice­creams of the same flavor = (Total number of combinations without any restriction) minus (the number of combinations where the ice creams are of same flavor.)

Using slot method, total number of icecreams can be obtained by one slot for each of the 4 icecreams as _*_*_*_ . Since three flavors are possible in each of these slots (four separate brands with three flavors each) the slots become 3*3*3*3 = 81

The number of combinations where the ice creams are of same flavor can be obtained again by 4 slots as used previously, which will be _*_*_*_ . Here, please note that, the first slot has the choice of 3 ice creams, whereas the following slots do not have such a choice and MUST be of the same flavor that was previously chosen in the first slot, whatever the flavor might be. This can be represented in the slot as 3*1*1*1 which equals 3. As mentioned above, the second, third and fourth slots have 1 in them because they do not have the choice of three flavors and are forced to follow whatever is the first icecream, while the first slot has 3 because it has the choice of 3 flavors.

And hence, 81 minus 3 gives your 78 which is OA.

Hope my explanation helps you at least in the slightest bit! :-D
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First up, we will find the total number of ways that Zachary can buy the ice-cream. He can choose 3 ice creams from each flavor. So, the number of ways that he can choose the four ice creams is 3 * 3 * 3 * 3 = 81

He should not choose the same flavors in all the ice creams and so, he cannot choose Vanilla from all the brands, chocolate from all the brands, and Strawberry from all the brands i.e, 3 ways of selecting the ice cream.

So, the total number of ways he can select ice creams is 81 - 3 = 78

Choice B is the answer.
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Deconstructing the Question

Zachary selects one ice-cream from each of 4 brands. Each brand offers 3 flavors.

So each selection has \(3\) choices, independently.

The restriction is that he cannot choose all ice-creams of the same flavor.

Step-by-step

Total possible selections without restriction:

\(3^4 = 81\)

Invalid cases are when all 4 ice-creams have the same flavor.

There are 3 such cases: all chocolate, all vanilla, or all strawberry.

\(3\)

Valid selections:

\(81 - 3 = 78\)

Answer: 78
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