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# When a certain coin is flipped, the probability that the coin will lan

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 6390
GMAT 1: 760 Q51 V42
GPA: 3.82
When a certain coin is flipped, the probability that the coin will lan  [#permalink]

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20 Apr 2017, 01:45
00:00

Difficulty:

55% (hard)

Question Stats:

54% (01:18) correct 46% (01:50) wrong based on 60 sessions

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When a certain coin is flipped, the probability that the coin will land on head or tail is 1/2 each. If the coin is flipped 4 times, what is the probability that it will land on tail at least twice on 4 flips?

A. $$\frac{3}{8}$$
B. $$\frac{1}{16}$$
C. $$\frac{1}{2}$$
D. $$\frac{5}{8}$$
E. $$\frac{11}{16}$$

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MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $99 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Manager Joined: 25 Apr 2016 Posts: 60 Re: When a certain coin is flipped, the probability that the coin will lan [#permalink] ### Show Tags 20 Apr 2017, 01:54 total no. of outcomes =16 favorable outcomes =(2h2t) 4!/2!*2! =6; (3t1h) 4!/3!=4; (4t) 4!/4!=1. Total favorable outcomes =11 req. probability=11/16-> E BSchool Forum Moderator Joined: 07 Jan 2016 Posts: 764 Location: India GMAT 1: 710 Q49 V36 Re: When a certain coin is flipped, the probability that the coin will lan [#permalink] ### Show Tags 20 Apr 2017, 02:50 It must land on tail atleast twice = 1- land of tail at most once HHHH = 1 THHH = 4 ways this can happen total ways = 1+4 Number of outcomes = 2^4 =16 P(at most once) = 5/16 P( atleast twice ) = 1 - P( at most once) =1 - 5/16 = 11/16 Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6390 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: When a certain coin is flipped, the probability that the coin will lan [#permalink] ### Show Tags 24 Apr 2017, 02:49 ==> In general, you solve probability questions using nCr combination. In other words, from TTHH, there are (4!/2!/2!)=6 possibilities, so you get (1/2)(1/2)(1/2)(1/2)*6=3/8. From TTTH, there are (4!/3!)=4 possibilities, so you get (1/2)(1/2)(1/2)(1/2)*4=1/4. From TTTT, there is only 1 possibility, so you get (1/2)(1/2)(1/2)(1/2)=1/16 and (3/8)+(1/4)+(1/16)=(6+4+1)/16=11/16. The answer is E. Answer: E _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$99 for 3 month Online Course"
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Re: When a certain coin is flipped, the probability that the coin will lan &nbs [#permalink] 24 Apr 2017, 02:49
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