
Solution:
Let ,
I be the event that a stock price increases,
D be the event that it decreases, and
R be the event that it remains the same.
For each stock, P(I) = P(D) = P(R) = 1/3
For each stock, any of these three cases may happen.
Therefore, total number a possible outcomes = 3 * 3 * 3=27.
Some of the outcomes are : (1,1,1), (I,I,D), (I,D,I), (D,I,I), (I,RR, (I,D,R) and so on.
P(at least two stocks increase in value) = P(only two stocks increase in value, and the other one either remains same or decreases) + P(all increase in value)
= 3 * 1/3 * 1/3 * 2/3 + (1/3)³
= 2/9 + 1/27
= 0.259