a. P(finding oil) = 1 - p(no oil) = 1 - 0.20 = 0.80
b. Test interpretation: We can see the conditional probability for the medium quality oil goes up while the other two go down. That means doing such positive test can improve your chance to find the medium quality oil.
New probability of finding oil is 0.8125 (see below)
0.5 * 0.2 + 0.2 * 0.8 / 0.5 * 0.2 + 0.2 * 0.8 + 0.3 * 0.2 = 0.8125
Send the money to the charity of Mr.Izzz's choice, since after all he is right in saying that you should
"do your own homework!!!!!!!!!!!!"
Is my answer right? Of course. I googled it. I have the source for this answer if needed.
