# Probability Random variables help

## Homework Statement

Can anyone help me with question 5d on this paper, I just dont get it.

I have done 5a,5b and 5c.

How do I find the values for x1 and x2 ?

http://www.mathspapers.co.uk/Papers/edex/S1Jan03Q.pdf [Broken]

Thanks.

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Let Y= X1 + X2

then we want the probability that Y=5

P(Y=5) = P(X1=2 , X2=3) + P(X1=3 , X2=3)

and since X1 and X2 are independent

P(Y=5) = P(X1=2)*P(X2=3) + P(X1=3)*P(X2=2)

and then just find the probabilites from the given probability distribution function

Last edited:
Let Y= X1 + X2

then we want the probability that Y=5

P(Y=5) = P(X1=2 , X2=3) + P(X1=3 , X2=3)

and since X1 and X2 are independent

P(Y=5) = P(X1=2)*P(X2=3) + P(X1=3)*P(X2=2)

and then just find the probabilites from the given probability distribution function
Oh I see that makes so much sense now, Thanks.