Probability of fraudulent phone calls

[JJ007]

It's found that 5% of the legitimate users make calls from two or more area codes in a single day. However, 30% of fraudulent users from two or more area codes in a single day. The proportion of fraudulent users is 1%. If a user makes calls from two or more area codes in a single day, what is the probability that the user is fraudulent?

I'm lost.

 

[tkhunny]

Several ways to go about it. The simplest probably would be a box.

1) Make a 2x2 box grid
2) Put lables down the right Fraudulent and Legitimate
3) Put labels across the top One Code and Two Codes
4) Extend the lines down and right to accomodate totals for each category.
5) Pick a number - 1000 often is convenient. Put this in the lower, right-hand corner
6) Use the information of 1% Fraudulent users to put a 10 (1% of 1000) in the top box above the 1000. This is the total of the "Fraudulent" row.
7) Use subtraction to find 990 for the total of the "Legitimate" row.
...
Use the other information to fill in the rest of the boxes.

You should then be able to answer most any question.