How good is a random sample?

A year group of 100 students has marks from 0 to 99, so the true mean is 49.5. You will draw samples by chance and see how close their means come. Then you compare small samples with large ones.

The idea. A random sample gives every student the same chance. Its mean is close to the true mean, and a larger sample is usually closer.

Your tasks.

1. In mean, return the mean of values.
2. In draw, return a random sample of size marks from marks.
3. Set size_b to 40, so that the second row of samples is four times as large.