Source Grockit: DS: Probability

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polter: Junior | Next Rank: 30 Posts; Posts: 16; Joined: Mon Dec 28, 2009 3:11 am

by polter » Wed Apr 25, 2012 6:38 pm

An ice cream truck is passing out free ice cream to each child at a playground. The driver randomly hands out only chocolate, vanilla and strawberry ice cream. What is the probability that a certain child will receive either chocolate or strawberry ice cream?

(1) The probability that a certain child will receive chocolate ice cream is 1/10.

(2) The probability that a certain child will receive vanilla is 1/4.

[spoiler]B[/spoiler]

I think since he is giving out randomly, then each will be equally likely event. Hence prob of chocolate = prob of vanilla = prob of strawberry.
Is this line of thinking incorrect?

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Source: Beat The GMAT — Data Sufficiency | Wed Apr 25, 2012 6:38 pm

shantanu86: Senior | Next Rank: 100 Posts; Posts: 59; Joined: Sun Mar 11, 2012 8:57 pmFollowed by: 1 member; Location: India; Thanked: 16 times

by shantanu86 » Wed Apr 25, 2012 7:35 pm

polter wrote:An ice cream truck is passing out free ice cream to each child at a playground. The driver randomly hands out only chocolate, vanilla and strawberry ice cream. What is the probability that a certain child will receive either chocolate or strawberry ice cream?

(1) The probability that a certain child will receive chocolate ice cream is 1/10.

(2) The probability that a certain child will receive vanilla is 1/4.

B

I think since he is giving out randomly, then each will be equally likely event. Hence prob of chocolate = prob of vanilla = prob of strawberry.
Is this line of thinking incorrect?

Hi polter,

Here the correct answer is [spoiler][/spoiler].

The driver has only three kinds of ice-cream and every single child gets one type.
Therefore,
P(Child gets ice-cream) = 1.

Now, the probability of getting a particular type can depend on a number of factors.. like quantity of each type present with the driver.

P(vanilla/ice-cream) = 1/4 .. indicates vanilla stock is 1/4th of total and rest is chocolate+vanilla.

Therefore,

P(reqd) = P(Child gets ice-cream)X (1-P(vanilla/ice-cream))
p(reqd) = 3/4

Hence, (2) alone is sufficient=> [spoiler] [/spoiler]is the correct answer.

Hope it helps!!

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