A jar contains 10 nickles and 5 dimes. Let X be the value of a coin drawn at random from this jar. Assuming all coins have the same probability to be drawn, find E(X) and Var(X).

Question

I understand how to find E(X) but not how to find Var(X).

Answer 1

You may use the computational formula for Var(X) = E(X²) - [E(X)]²

Since you already calculated for E(X), you just need to calculate E(X²). To do this, go back on how you calculated E(X) and instead of X, replace it by X², i.e. in your calculation instead of 10 and 5, use 10² and 5²