The more complex stuff I get, why did I get stuck on this?

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ithamarsorek: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Tue Dec 07, 2010 1:01 pm

by ithamarsorek » Thu Dec 09, 2010 2:41 am

Anyone... I need an simple explanation on how to solve this step by step.
For some reason it does not sink

When tossing a fair coin 4 times what is the likelihood of having at least 3 heads?

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Source: Beat The GMAT — Problem Solving | Thu Dec 09, 2010 2:41 am

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Dec 09, 2010 3:30 am

ithamarsorek wrote:When tossing a fair coin 4 times what is the likelihood of having at least 3 heads?

The event "at least 3 heads" contains the following events:

1. Exactly 3 heads.
2. Exactly 4 heads.

Possible exactly 3 heads: (HHHT), (HHTH), (HTHH) and (THHH)
Each have a probability 1/16 => Total = 4*(1/16) = 1/4

Possible exactly 4 heads: (HHHH) => Probability = 1/16

Required probability = (1/4) + (1/16) = 5/16

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