# Numbered balls and replacement

1. Mar 25, 2013

### ParisSpart

we have numbered balls in a bag from 1 to 10. We put our hand and take one. Let the number of X be the first number of the ball. Replace the ball into the bag again and takes another ball at random. Let the number of Y the number of the second ball.

IF Z =max{X, Y}.

we want to find P(Z=9)

First of all i used the type (n+k-1)c(k) because we replace the ball again and i found this (8)c(55) but its says tha its not correct answer, what i am doing wrong?

2. Mar 25, 2013

### jashua

There are 17 possible ways you choose 2 balls with replacement such that Z=max(X,Y)=9. The number of all possible ways you choose 2 balls with replacement is 100. Now you should be able to find the answer.

Last edited: Mar 25, 2013