may 2015 Math HL paper 1

[brianb97]

TZ1

What was that probability question. 5 games, if lose 3 consecutively the coach is out. prob (win for a specific game) is 1/2 prob (draw) is 1/6, prob (lose) is 1/3. Results of one game independent of the others Find P (coach says bye bye)

I didn't get get it in the exam but after paper 2 i tried to solve it i got 21/243 (or 7/81)

There were like 20 answers among my grade

Lol hated that question. Usually probably is super straight-foward, but that question got me like 

[Rahul]

TZ1

What was that probability question. 5 games, if lose 3 consecutively the coach is out. prob (win for a specific game) is 1/2 prob (draw) is 1/6, prob (lose) is 1/3. Results of one game independent of the others Find P (coach says bye bye)

I didn't get get it in the exam but after paper 2 i tried to solve it i got 21/243 (or 7/81)

There were like 20 answers among my grade

 

My solution was as follows, could be wrong though!

 

Let L denote a loss, W denote a win, and X denote any outcome.

 

Then there are three separate ways in which the coach can be fired:

1: {L,L,L,X,X}

2: {X,L,L,L,X}

3: {X,X,L,L,L}

 

The probability of each option is as follows:

P(Case 1)=P(Case 2)=P(Case 3)

=(P(X))²(P(L))³

=1/27

 

Thus:

P(Coach Fired)=P(Case 1)+P(Case 2)+P(Case 3)

=1/9

[kw0573]

 

TZ1

What was that probability question. 5 games, if lose 3 consecutively the coach is out. prob (win for a specific game) is 1/2 prob (draw) is 1/6, prob (lose) is 1/3. Results of one game independent of the others Find P (coach says bye bye)

I didn't get get it in the exam but after paper 2 i tried to solve it i got 21/243 (or 7/81)

There were like 20 answers among my grade

 

My solution was as follows, could be wrong though!

 

Let L denote a loss, W denote a win, and X denote any outcome.

 

Then there are three separate ways in which the coach can be fired:

1: {L,L,L,X,X}

2: {X,L,L,L,X}

3: {X,X,L,L,L}

 

The probability of each option is as follows:

P(Case 1)=P(Case 2)=P(Case 3)

=(P(X))²(P(L))³

=1/27

 

Thus:

P(Coach Fired)=P(Case 1)+P(Case 2)+P(Case 3)

=1/9

 

But then you are over counting XLLLL and LLLLX, and then there's LLLLL all to be taken into consideration