The probability of a man hitting a target is 2/3. If he doesn't stop shooting until he hits the target for the first time, a) What is the probability of taking 5 shots to hit the target? b) Which is the least number of shots needed in order to have a probability (of hitting the target) greater than 0.95?

Keenan Conway

Keenan Conway

Answered question

2022-09-24

Geometric distribution with given probability value.
The probability of a man hitting a target is 2/3. If he doesn't stop shooting until he hits the target for the first time, a) What is the probability of taking 5 shots to hit the target? b) Which is the least number of shots needed in order to have a probability (of hitting the target) greater than 0.95?

Answer & Explanation

Emaidedip6g

Emaidedip6g

Beginner2022-09-25Added 11 answers

Step 1
We want the probability of not hitting to be < 0.05. The probability of not hitting in x trials is ( 1 3 ) x . We want this to be less than 0.05.
Step 2
One does not really need theory to find the answer, just a bit of fooling around with the first few powers of 3. But if we want to use logarithms, we have ( 1 3 ) x < 0.05 if and only if x ln ( 1 / 3 ) < ln ( 0.05 ). Now we can find by calculator the solution of x ln ( 1 / 3 ) = ln ( 0.05 ) and round suitably. (Logarithms to any base will do. You may prefer logarithms to the base 10.)

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