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Suppose that the longevity of a light bulb is exponential with a mean lifetime of eight years.

a. Find the probability that a light bulb lasts less than one year.

b. Find the probability that a light bulb lasts between six and ten years.

c. Seventy percent of all light bulbs last at least how long?

d. A company decides to offer a warranty to give refunds to light bulbs whose lifetime is among the lowest two percent of all bulbs. To the nearest month, what should be the cutoff lifetime for the warranty to take place?

e. If a light bulb has lasted seven years, what is the probability that it falls within the 8th year.

Short Answer

Expert verified

a). P(x<1)=0.1175is the probability that a light bulb will last less than one year.

b). The probability that a light bulb will last six to 10years

P(6<x<10)=0.1859.

c). The value of P(x>k)=2.85years.

d). The value P(x<k)is 0.1616.

e). The value ofP(x<8∣x>7)=0.1175.

Step by step solution

01

Part (a) Step 1: Given Information

Given data:

μ=8,

m=0.125

02

Part (a) Step 2: Explanation

The exponential distribution is defined as follows:

P(X<x)=1-e-mx

The required probability can be estimated using the formula,

P(x<1)=1-e-0.125×1

P(x<1)=1-0.8825

P(x<1)=0.1175

P(x<1)=0.1175is the probability that a light bulb will last less than one year.

03

Part (b) Step 1: Given Information

Given data:

μ=8

m=0.125

04

Part (b) Step 2: Explanation

The exponential distribution is defined as follows:

P(X<x)=1-e-mx

The required probability can be estimated using the formula:

P(6<x<10)=P(x<10)-P(x<6)

P(6<x<10)=1-e-0.125×10-1-e-0.125×6P(6<x<10)=0.4784-0.2865

P(6<x<10)=0.1859

05

Part (c) Step 1: Given Information

Given data:

μ=8,

m=0.125

06

Part (c) Step 2: Explanation

The exponential distribution is defined as follows:

P(X<x)=1-e-mx

P(x>k)=1-1-e-0.125×k

localid="1653537354167" 0.70=e-0.125×k

Taking log on both sides,

In(0.70)=Ine-0.125×k

-0.125k=-0.3567

k=2.85

07

Part (d) Step 1: Given Information

Given data:

μ=8

m=0.125

08

Part (d) Step 2: Explanation

The given probability is P(x<k)

P(x<k)=1-e-0.125k

0.02=1-e-0.125k

e-0.125k=0.98

Taking log on both sides,

e-0.125k=0.98

-0.125k=-0.0202

k=0.1616

09

Part (e) Step 1: Given Information

The given probability is

P(x<8∣x>7)=1-P(x<8∣x>7).

10

Part (e) Step 2: Explanation

The memory less exponential distribution can be calculated as follows:

P(x<t+r∣x>t)=P(x>r)P(x<8∣x>7)=1-P(x>7+1∣x>7)P(x<8∣x>7)=1-P(x>1)P(x<8∣x>7)=1-[1-P(x>1)]

P(x<8∣x>7)=P(x<1)

P(x<8∣x>7)=1-e0.125×1

P(x<8∣x>7)=1-0.8825

P(x<8∣x>7)=0.1175

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