<s>[INST] This problem is based on the fact that the expected value of a random variable, X, is equal to the sum of the products of each possible outcome and its probability of occurring. Using this definition: 
$$ E(X) = \sum_{i=1}^{n} x_i P(x_i) $$

A certain stock either goes up, stays the same, or goes down every month. If the probabilities of it going up, staying the same, or going down are 0.4, 0.3, and 0.3, respectively, and if the respective changes in the stock price are +20, 0, and -15, what is the expected monthly change in the stock price?[/INST]