Probabilitical Steps..

A man takes a step forward with probability 0.4 and backward with probability 0.6 . Find the Probability at the end of eleven steps he is one step away from starting point.

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The answer is 0.37.

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2 solutions

Since the man is one step away from starting point it means that either

(i) man has taken 6 steps forward and 5 steps backward.

(ii) man has taken 5 steps forward and 6 steps backward.

Taking movement of 1 step forward as success and movement of 1 step backward as failure.

p = Probability of success = 0.4

q = Probability of failure = 0.6

Applying Binomial Theoram, R e q u i r e d p r o b a b i l i t y = P ( X = 6 t o X = 5 ) Required \ probability = P(X=6 \ to \ X = 5) = P ( X = 6 ) + P ( X = 5 ) = P(X = 6) + P(X = 5) = ( 11 6 ) p 6 q 5 + ( 11 5 ) p 5 q 6 = {11 \choose 6}p^6q^5 + {11 \choose 5}p^5q^6 = ( 11 5 ) ( p 6 q 5 + p 5 q 6 ) = {11 \choose 5}(p^6q^5 + p^5q^6) = 11.10.9.8.7 1.2.3.4.5 [ ( 0.4 ) 6 ( 0.6 ) 5 + ( 0.4 ) 5 ( 0.6 ) 6 ] = \frac{11.10.9.8.7}{1.2.3.4.5}[(0.4)^6(0.6)^5 + (0.4)^5(0.6)^6] = 0.37 = \boxed{0.37}

Melissa Quail
Feb 4, 2015

The man has to finish one step away from starting point so he has either taken:

5 steps forward and 6 steps back

Or 6 steps forward and 5 steps back

For each arrangement of 5 steps forward and 6 steps back, the probability of that happening is:

( 0.4 ) 5 (0.4)^5 x ( 0.6 ) 6 (0.6)^6

And there are 11C5 ways to place the 5 forward steps within the 11 steps so the total probability for this case is:

( 0.4 ) 5 (0.4)^5 x ( 0.6 ) 6 (0.6)^6 x (11C5)

And for each arrangement of 6 steps forward and 5 steps back the probability is:

( 0.4 ) 6 (0.4)^6 x ( 0.6 ) 5 (0.6)^5

And there are 11C6 ways to place the 6 forward steps (which is equal to 11C5 ways to place the backward steps) so the probability of this case is:

( 0.4 ) 6 (0.4)^6 x ( 0.6 ) 5 (0.6)^5 x (11C6)

So the total probability is the two cases added together which equals:

( ( 0.4 ) 5 ((0.4)^5 x ( 0.6 ) 6 (0.6)^6 x (11C5)) + ( ( 0.4 ) 6 (0.4)^6 x ( 0.6 ) 5 (0.6)^5 x (11C6)) = 0.37 \boxed{0.37} to 2dp.

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