Question

A jar contains 8 red marbles and y white marbles. If Joan takes 2 random marbles from the jar, is it more likely that she will have 2 red marbles than that she will have one marble of each color?
(1) y ≤ 8
(2) y ≥ 4

Answer 1

Answer:

a) y </= 8 is not sufficient

b) y >/= 4 is sufficient (y is not less than 3.5)

Step-by-step explanation:

Number of Red marbles = 8

Number of white marbles = y

Total number of marbles in the jar = 8+ y

Let Pr(R) be the probability of picking red marbles

Let Pr(W) be the probability picking white marbles

Pr(R) = 8/ (8+y)

Pr(W) = 7/(7+y)

Pr(RR) = Pr(R1) * Pr( R2)

= 8/(8+y) * 7/(7+y)

Pr(RW) = Pr(R1) * Pr(W2) + Pr(W1) * Pr(R2)

= 2[Pr(R1) * Pr(W2)

= 2[8/(8+y) * y/(7+y)]

The probability of having 2 red is greater than one marble of each color.

Pr(RR) > Pr( RW)

8/(8+y) * 7/(7+y) > 2[8/(8+y) * y/(7+y)]

7/(7+y) > 2(y/(7+y)

7/y > 2

7/2 > y

3.5 > y

y < 3.5

Therefore;

a) y </= 8 is not sufficient

b) y >/= 4 is sufficient (y is not less than 3.5)