Question

Does crime pay? The FBI Standard Survey of Crimes showed that for about 80% of all property crimes (burglary, larceny, car theft, etc.), the criminals are never found and the case is never solved†. Suppose a neighborhood district in a large city suffers repeated property crimes, not always perpetrated by the same criminals. The police are investigating five property crime cases in this district.

1. What is the expected number of crimes that will be solved? (Round your answer to two decimal places.)

What is the standard deviation? (Round your answer to two decimal places.)

2. How many property crimes n must the police investigate before they can be at least 90% sure of solving one or more cases?

Answer 1

1.

What is the expected number of crimes that will be solved?

n*(1-p) = 5*(1-0.8) = 1

What is the standard deviation?

sd = [n*p*(1-p)]^0.5 = [5*0.2*0.8]^0.5 = 0.89

2.

let n be rquired number

so X is nmber of cases solved

X~Bin(n,0.2)

we have to find n so that

P(X>1) >0.90

now

E(x) =0.2n

now

this implies

now from standard normal distribution

this gives

this gives

n=15 ans