coin

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ketkoag: Legendary Member; Posts: 876; Joined: Thu Apr 10, 2008 8:14 am; Thanked: 13 times

by ketkoag » Tue Jun 09, 2009 7:12 am

A coin is tossed 3 times. What is the probability that it will be up with the same side in all the 3 times?

[spoiler]please explain how answer is ¼..[/spoiler]

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Source: Beat The GMAT — Problem Solving | Tue Jun 09, 2009 7:12 am

sacx: Master | Next Rank: 500 Posts; Posts: 139; Joined: Wed Oct 22, 2008 4:36 am; Thanked: 17 times

by sacx » Tue Jun 09, 2009 7:22 am

total possibilities = 8

HHH
HHT
HTH
THH
TTT
TTH
THT
HTT

same side up = 2 (HHH,TTT)

probability = 2/8 = 1/4

SACX

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yogami: Master | Next Rank: 500 Posts; Posts: 170; Joined: Tue May 26, 2009 12:00 pm; Thanked: 5 times

by yogami » Tue Jun 09, 2009 10:33 am

in order to get same side up all three times you need to get HHH or TTT
Hence the probability is (1/2)^3 + (1/2) ^3 = 1/4

200 or 800. It don't matter no more.

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