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this will probably be fairly easy for some of you, but i'm [#permalink]

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26 Jul 2004, 12:03

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this will probably be fairly easy for some of you, but i'm stuck on it:

a fair coin with sides marked heads and tails is to be tossed eight times. what is the probability that the coin will land tails side up more than five times?

IMO, formulas are good, but not good enough. do try to understand why the formula works the way it works. ETS makes a living out of making simple problems look difficult. If just one thing is out of place, it wouldnt be difficult to panic.

Search for explanations by anupag/akamai brah/stolyar for this.

1. Total no. of outcomes is 2^8. You are flipping 8 times and probability of tail showing up everytime you flip the coin is 1/2. When you do it eight times it is 1/2^8. Or the total outcomes (denominator) is 2^8.

2. Now, the question is what is the probability of tails coming up atleast 5 times - this means that tails can appear 5 times, 6 times, 7 times or all the 8 times. Think combinations and you'll get 8C5 + 8C6 + 8C7 + 8C8 ways of getting atleast 5 tails and this adds up to 93.

is there any easy way of handling this....calculating the probability in this case is cumbersome...is this a GMAT Type anyway??

I agree with smandalika and venksune on this one. However, the question asks for the "probability that the coin will land tails side up more than five times". I don't think that includes 5 times, so I only did it with 6, 7, and 8 times. My answer: 37/256.

is there any easy way of handling this....calculating the probability in this case is cumbersome...is this a GMAT Type anyway??

I agree with smandalika and venksune on this one. However, the question asks for the "probability that the coin will land tails side up more than five times". I don't think that includes 5 times, so I only did it with 6, 7, and 8 times. My answer: 37/256.