Expected value and a simulated game

You will write a function for the expected value of any game, then simulate a made-up game many times and check that the average gain lands near the expected gain.

The idea. The expected value is the sum of each value times its probability. A game can be simulated by picking a value with the right probabilities: random.choices(values, weights=probs)[0] does exactly that. By the law of large numbers, the average of many plays settles near the expected value.

Your tasks.

1. Finish expected_value(values, probs). Return the sum of each value times its probability.
2. Finish play(values, probs). Return one value picked at random with the given probabilities.
3. Finish average_gain(values, probs, cost, n). Play n times and return the average of the value minus the cost.