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A die is continually rolled until the total sum of all rolls exceeds 300. Approximate the probability that at least 80 rolls are necessary.

Short Answer

Expert verified

P{Y79≤300}=0.9429

Step by step solution

01

Step 1. Given information

Let Xidenote the value of the die, of ithroll.

02

Step 2. Mean and variance of Xi

Mean = E(Xi)=(1+2+3+4+5+6)6=216=72

Variance =role="math" localid="1648052296820" V(Xi)=3512

03

Step 3. Introducing Yn

Assuming that die is continually rolled until the total sum of all rolls exceeds 300.

Let Ynrepresents the total sum of all rolls with n number of rolls.

Yn=∑Xi1n

04

Step 4. Mean and variance of Yn

Mean = E(Yn)=n×E(Xi)=7n2

Variance =V(Yn)=n×V(Xi)=35n12

05

Step 5. To find

The probability that at least 80 rolls are necessary can be written as -

P∑Xi≤300179=PY79≤300

06

Step 6. Using central limit theorem

P{Y79≤300}=PY79-7(79)235(79)12≤300-7(79)235(79)12P{Y79≤300}=PZ≤1.58P{Y79≤300}=0.9429

07

Step 7. Final answer

The probability that at least 80 rolls are necessary is 0.9429

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