An urn contains three grey mice and one white mouse. A cat puts her paw into the urn and pulls out mice until she gets a grey mouse. What is the expected number of times the cat puts her paw into the urn? (The cat does not return any mice to the urn.) Solution E(X)= n*P(X)= P(G in first turn)*1 + P(W and G in sucessive turns)*2 = 3/4 + 2/4 = 5/4.