Suppose that X is a normal random variable with mean 5. If P(X>9)=0.2, approximately what is Var(X)?

suppose that x is a normal random variable with mean 5

This question aims to find the probability of a normally distributed random variable X. A random variable is one whose value is determined by the results of a statistical experiment.

The normal distribution, also known as the Gaussian distribution or the z-distribution, has a mean of zero and a standard deviation of one. Data in a normal distribution is symmetrically distributed and has no skew. The data takes the shape of a bell when plotted on a graph, with most values grouping around a central region and scattering off as they move away from the center.

The two characteristics such as mean and standard deviation define the graph of the normal distribution. The mean/average is the maximum of the graph, whereas the standard deviation measures the amount of spread away from the mean.

Expert Answer

Let μ and σ be the mean and standard deviation of the random variable X. According to the question:

μ=5, P(X>9)=0.2  and  we have to find  Var(X) =σ2.

Since, P(X>9)=0.2

P(X<9)=10.2=0.8

P(Z<xμσ)=0.8

P(Z<95σ)=0.8

ϕ(95σ)=0.8

So, by inverse use of the z table, when ϕ(z)=0.8 then z0.84. And hence:

95σ=0.84

4σ=0.84

σ=40.84=4.76

Therefore,  Var (X) =σ2=(4.76)2=22.66

Example 1

Consider X as a normally  distributed random variable with μ=22 and σ=3. Find P(X<23), P(X>19)  and  P(25<X<30).

Solution

Here, μ=22 and σ=3

Therefore,  P(X<23)=P(Z<Xμσ)

P(Z<23223)=P(Z<0.33)=0.6293

Now, P(X>19)=P(Z>Xμσ)

P(Z>19223)=P(Z>1)

P(Z>1)=1P(Z<1)=10.1587=0.8413

Also,  P(25<X<30)=P(25223<Z<30223)

P(1<Z<2.66)=P(0<Z<2.66)P(1<Z<0)=0.99610.8413=0.1548

Geogebra export

Area under the normal curve between 25 and 30

Example 2

The time between battery charges for some specific types of computers is normally distributed, with a mean of 30 hours and a standard deviation of 12 hours.  Alice has one of these computer systems and is curious about the likelihood that the time will be between 60 and 80 hours.

Solution

Here, μ=30  and σ=12

To find: P(60<X<80)

Now, P(60<X<80)=P(603012<Z<803012)

P(2.5<Z<4.16)=P(0<Z<4.16)P(2.5<Z<0)

=0.49980.4938=0.0060

Example 3

A normal distribution model with a mean of 6 cm and a standard deviation of 0.03 cm is used to approximate the length of similar components produced by a company. If one component is selected randomly, what is the likelihood that this component’s length is between 5.89 and 6.03 cm?

Solution

Given,  μ=6 and σ=0.03

To find: P(5.89<X<6.03)

Now, P(5.89<X<6.03)=P(5.8960.03<Z<6.0360.03)

P(3.66<Z<1)=P(3.66<Z<0)+P(0<Z<1)

=0.0002+0.8413=0.8415

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