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# Need help

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Joan tries to solve a really hard problem once each day. She has a 1/4 probability of solving it each day. What is the probability that she will solve it before her sixth try?

Oct 4, 2019

#1
+19879
0

BEFORE her 6th try is 5 tries

The probability that she DOESN"T solve it daily is 3/4

1- (3/4)5 = 781/1024 = 76.3%      ???

Oct 4, 2019
edited by Guest  Oct 4, 2019
edited by Guest  Oct 4, 2019

#1
+19879
0

BEFORE her 6th try is 5 tries

The probability that she DOESN"T solve it daily is 3/4

1- (3/4)5 = 781/1024 = 76.3%      ???

ElectricPavlov Oct 4, 2019
edited by Guest  Oct 4, 2019
edited by Guest  Oct 4, 2019
#2
+107116
+1

Joan tries to solve a really hard problem once each day. She has a 1/4 probability of solving it each day. What is the probability that she will solve it before her sixth try?

EPs way is the best way.

Here is another method though

Prob that she solves it on her first try    =1/4

Prob that she solves it on her second try = 3/4*1/4 = 3/16

Prob that she solves it on her third try = (3/4)^2   *  1/4   =  9/64

Prob that she solves it on her forth try = (3/4)^3   *  1/4   =  27/256

Prob that she solves it on her f5th try = (3/4)^4   *  1/4   =  81/1024

1/4(1+3/4+9/16+27/64+81/256) = 0.7626953125 equals a approx  76.3%

Oct 5, 2019