# PmF

• Dec 6th 2009, 12:55 PM
durham2
PmF
A fair coin is tossed four times, and the random variable X is the number of heads in the first three tosses and the random variable Y is the number of heads in the last three tosses.

Find the joint probability mass function and marginal probability mass function of X and Y.

ON this problem, I did HHHH, HHHT, HHTH,....and so on. Is that how you start off and make a table from there. like there is a 1/13 chance of getting first three heads? I am not completely sure.
• Dec 6th 2009, 06:25 PM
mr fantastic
Quote:

Originally Posted by durham2
A fair coin is tossed four times, and the random variable X is the number of heads in the first three tosses and the random variable Y is the number of heads in the last three tosses.

Find the joint probability mass function and marginal probability mass function of X and Y.

ON this problem, I did HHHH, HHHT, HHTH,....and so on. Is that how you start off and make a table from there. like there is a 1/13 chance of getting first three heads? I am not completely sure.

List all the outcomes and use these to set up a table with X along the top and Y along the side. By the way, probability that the first three are heads is Pr(HHHT) + Pr(HHHH) = 1/16 + 1/16 = 1/8 ....
• Dec 8th 2009, 07:55 PM
durham2
Quote:

Originally Posted by mr fantastic
List all the outcomes and use these to set up a table with X along the top and Y along the side. By the way, probability that the first three are heads is Pr(HHHT) + Pr(HHHH) = 1/16 + 1/16 = 1/8 ....

okk so you put all 16 outcomes in a table? or is it 0-4 for how many heads you can get?
• Dec 9th 2009, 03:01 AM
mr fantastic
Quote:

Originally Posted by durham2
okk so you put all 16 outcomes in a table? or is it 0-4 for how many heads you can get?

If
Quote:

X is the number of heads in the first three tosses
then what is the minimum and maximum possible value of X?

If
Quote:

the random variable Y is the number of heads in the last three tosses
then what is the minimum and maximum possible value of Y?
• Dec 9th 2009, 03:11 AM
coder
Quote:

Originally Posted by durham2
okk so you put all 16 outcomes in a table? or is it 0-4 for how many heads you can get?

For X,Y seperately 0-3 might be sufficient.