Answer:
14.7
Step-by-step explanation:
Total number of grades= 6
Imagine Y to be number of papers till we get all grades once. Hence
Yi= Number of papers till we get i <u>th</u> newer grades
Expected value of Y₆= ?
The difference between getting a new grade maybe represented as
Xi= Yi+1 - Yi
Using above equation for Y₆, we get
[Y₆]= ∑⁵i=o Xi
which means, we need to get 5 different grades from the first grade.
Number of tries to see second new grade maybe represented as
X₁= {(6-1)/6}, which, for generalization is written as Xi=geo{(6-i)/6}
Xi represents the success probability of seeing further new grade.
Expected value of Xi is inverse of parameter of geometric distribution, which is
[Xi] = 6/(6-i) = 6.{1/(6-i)}
Expected value of Y₆= [∑⁵ i=0 Xi] = ∑⁵ i=0 [Xi]
Substituting value of [Xi] in the above expression
6.∑⁵i=0 {1/(6-i)} = 6. ∑⁶i=1 (1/i)
Now solving for 6 grades
Y₆ = 6[(1/6) + (2/6) + (3/6) + (4/6) + (5/6) + (6/6)]
Y₆ = 6 x 2.45 = 14.7
