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Two terms to keep in mind :

a) Probability density function is used where we have to find probability at each point on the real number line and hence continuous probability..

b) Probability mass function(also known as probability moment function) is used where probability is found at discrete points only (normally at integral points) and hence discrete probability..

Given , probability mass function ; p(n)  =  2-n  =  1 / (2n)

Now given X and Y are 2 random variables ..

So we need to find P(X >= 2Y)..For this we need to calculate systematically..

Say  Y =  1 , so X can take any values from 2 onwards..

Hence P(Y = 1 and X >= 2)  =  P(Y = 1)  . P(X >= 2)   [ As X and Y are independent random variables ]

                                          =  P(Y = 1) . [ P(X = 2) or P(X = 3) .............]

                                          =  1/2 . [ 1/22  +  1/23  + ......... ]

                                          =  1/22

Similarly when Y =  2

          P(Y = 2 and X >= 4)   =  P(Y = 2)  . P(X >= 4)   [ As X and Y are independent random variables ]

                                          =  P(Y = 2) . [ P(X = 4) or P(X = 5) .............]

                                          =  1/22 . [ 1/24 +  1/25  + ......... ]

                                          =  1/2.  1/23

Similarly when Y =  3          

           P(Y = 3 and X >= 6)   =  P(Y = 3)  . P(X >= 6)   [ As X and Y are independent random variables ]

                                          =  P(Y = 3) . [ P(X = 6) or P(X = 7) .............]

                                          =  1/23 . [ 1/2+  1/27  + ......... ]

                                          =  1/23 .  1/25

                                          =  1/22 . 1/26

Hence  P(X >= 2Y)              =  P(Y = 1 and X >= 2)  or P(Y = 2 and X >= 4) or P(Y = 3 and X >= 6) .............

                                          =  1/22 . 1  +  1/22 . 1/23  +  1/22 . 1/26  + .............

                                          =  1/22 [  1 + 1/23 + 1/26 + ........... ]

                                          =  1/22 .  1 / (1 - 1/23)

                                          =  (1/4) . (8/7)

                                          =  2/7

                                          =  0.29 [ correct to 2 decimal places ]

Hence P(X >= 2Y)               =  0.29

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