91Ó°ÊÓ

The object is to calculate the proportion of all non-California households earning more than $\$250,000$per year using the given information. $12\%$of all U.S. households are in California and $1.3\%$of all U.S. households earn more than $\$250,000$per year. Also,$3.3\%$all California households earn more than$\$250,000$ per year.

The proportion of all U.S. households earn more than $\$250,000$per year is

$P(>250K)=\frac{1.3}{100}$

The proportion of households in California is $P\left(C\right)=\frac{12}{100}$

The proportion of all non-California households earning more than$\$250,000$per a year is $P(>250Kâˆ\pounds \stackrel{¯}{C})$

The proportion of all U.S. households earning more than $\$250,000$per year can be shows using the BAYES's formula as

$P(>250K)=P\left(C\right)P(>250Kâˆ\pounds C)+P\left(\stackrel{¯}{C}\right)P(>250Kâˆ\pounds \stackrel{¯}{C})$

Put the values and simplify to get the proportion of all non-California households earning more than$\$250,000$

$\frac{1.3}{100}=\left[\frac{12}{100}×\frac{3.3}{100}\right]+\left[\frac{88}{100}×P(>250Kâˆ\pounds \stackrel{¯}{C})\right]$

$0.013=[0.12×0.033]+[0.88×P(>250Kâˆ\pounds \stackrel{¯}{C}\left)\right]$

$P(>250Kâˆ\pounds \stackrel{¯}{C})=\frac{0.013-0.00396}{0.88}$

$=0.01027$

The proportion of all non-California households earning more than per year is$0.01027$

We have to give that a randomly chosen U.S. household earns more than $\$250,000$per year, then the probability it is a California household is$P(Câˆ\pounds >250K)$

We have the conditional probability, the event$P(Câˆ\pounds >250K)$can be expressed as

$P(Câˆ\pounds >250K)=\frac{P(Câˆ\copyright >250K)}{P(>250K)}$

$=\frac{P\left(C\right)P(>250Kâˆ\pounds C)}{P(>250K)}$

Substitute and simplify values to get the required probability.

Given that a randomly chosen U.S. household earns more than$\$250,000$per year, then the probability it is a California household is

$P(Câˆ\pounds >250K)=\frac{P\left(C\right)P(>250Kâˆ\pounds C)}{P(>250K)}$

$=\frac{\frac{12}{100}×\frac{3.3}{100}}{\frac{1.3}{100}}$

$=0.3046$

Finally we get at given randomly chosen U.S. household that earns more than $\$250,000$per year, then the probability it is a California household is $0.3046$