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Probability

Question
CBSEENMA12035720

Two numbers are selected at random (without replacement) from the first six positive integers. Let X denote the larger of the two numbers obtained. Find the probability distribution of the random variable X, and hence find the mean of the distribution.

Solution

If 1 is the smallest number,
the other numbers are:2,3,4,5,6
If 2 is the smallest number, the other numbers are:3,4,5,6
If 3 is the smallest number, the other numbers are:4,5,6
If 4 is the smallest number, the other numbers are: 5, 6
If 5 is the smallest number, the other number is:6
Thus comma space the space sample space space space is space straight S space equals space open curly brackets table row cell 12 comma space 13 comma space 14 comma space 15 comma space 16 end cell row cell 23 comma space 24 comma space 25 comma space 26 end cell row cell 34 comma space 35 comma space 36 end cell row cell 45 comma space 46 end cell row 56 end table close curly brackets
Thus, there are 15 set of numbers in the sample space. 
Let X be
X:           2             3                  4                 5                   6
straight P left parenthesis straight X right parenthesis colon      1/15        2/15             3/15            4/15               5/15
We know that,
straight E left parenthesis straight X right parenthesis space equals space straight X subscript straight i straight P left parenthesis straight X subscript straight i right parenthesis
equals space 2 cross times 1 over 15 plus 3 cross times 2 over 15 plus 4 cross times 3 over 15 plus 5 cross times 4 over 15 plus 6 cross times 5 over 15
equals fraction numerator 2 plus 6 plus 12 plus 20 plus 30 over denominator 15 end fraction
equals 70 over 15
almost equal to space 4.66

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