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Retain the exceptional all-blue score in the conditional game

A box contains 2 red balls,3 green balls and x blue balls, with integer x≥5x\ge5. A contestant chooses 5 balls without replacement. S is the number of green balls plus three times the number of red balls. Blue balls score nothing unless all 5 balls are blue, in which case S=25.

(i) Show that P(S=6)=20x(x2−3x+14)(x+5)(x+4)(x+3)(x+2)(x+1)P(S=6)=\dfrac{20x(x^2-3x+14)}{(x+5)(x+4)(x+3)(x+2)(x+1)}. marks

(ii) Given P(S=6)=5/63P(S=6)=5/63, calculate x. marks

(iii) Complete the distribution table. marks

sGiven P(S=s)P(S=s)
1to find
25/425/42
35/635/63
4to find
55/215/21
65/635/63
7to find
85/845/84
91/2521/252
25to find

(iv) Evaluate E(S)E(S) and find P(S>E(S))P(S>E(S)). marks

(v) Find the probability that no green balls are drawn, given S>E(S)S>E(S). marks

[2017/CJC/II/10]

CJC · Review problems · Question 9

The all-blue draw is an exceptional score 25, not score 0.

Read the worked solution