A new promotion at Insomnia Cookies lets you play the following game: by paying a fixed amount of money, you can get a free cookie every day, selected uniformly at random from chocolate chunk, double chocolate mint, and peanut butter chip, until you get a chocolate chunk (after which the promotion will stop). Suppose you decide to play this game.
(a)Let X be the number of free cookies you obtain with this promotion. Find Ex(X).
(b)Let Y be the number of free cookies you get that are not chocolate chunk. Find Ex(Y).
a. X is the number of free cookies I get.
So X takes the values 1,2,3,4,5,......
Now,
P(X=1)=1/3. I get the chocolate chunk.
P(X=2)=2/3*1/3. I get any one of double chocolate mint or peanut butter chip on first day and chocolate chunk the next day.
Similarly,
P(X=3)=2/3*2/3*1/3
So, E(X)=1*1/3+2*2/3*1/3+3*2/3*2/3*1/3+....
Now Y takes the values 0,1,2,3,...
P(Y=0)=1/3. You get chocolate chunk at first attempt.
P(Y=1)=2/3*1/3. You miss the first time and get chocolate chunk in the second time. and so on.
So E(Y) equals,
E(Y)=1*2/3*1/3+2*2/3*2/3*1/3+...
Get Answers For Free
Most questions answered within 1 hours.