Question

Let represent the number of typographical errors made per page typed by a receptionist during a particular day at the office. The following table lists the probability distribution of .
= 0 1 *** 4 5 ( = ) * ** 0.40 0.13 0.02
Assuming that * = 2 (**) and E(X) = 12.77
(a) Find the values of ‘*’, ‘**’, and ‘***’.
(b) Determine P(1 ≤ < 4).

Answer 1

Using the principle of expected value and discrete probability distribution, the missing values are :

• * = 0.30
• ** = 0.15
• *** = 30
• P(1 ≤ Y ≤ 4) = 0.28

The Expected value, E(X) is defined thus :

• E(X) = Σ[(X) × (P(X)]

The cummulative sum of the probability is 1 :

• (* + ** + 0.40 + 0.13 + 0.02) = 1 - - - (1)
• * = 2(**)

Hence, we have ;

2** + ** + 0.40 + 0.13 + 0.02 = 1

3** + 0.55 = 1

3** = 1 - 0.55

3** = 0.45

** = 0.45 / 3

** = 0.15

Hence,

* = 2(0.15)

* = 0.30

To find *** :

E(X) = (0 × 0.30) + (1 × 0.15) + (*** × 0.40) + (4 × 0.13) + (5 × 0.02)

12.77 = 0 + 0.15 + 0.40*** + 0.52 + 0.10

12.77 = 0.77 + 0.40***

12.77 - 0.77 = 0.40***

12.00 = 0.40***

*** = 12.00/0.40

*** = 30

B.)

P(1 ≤ Y ≤ 4) = P(y = 1) + P(y = 4)

P(1 ≤ Y ≤ 4) = 0.15 + 0.13 = 0.28

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