The three averages by computer

You will write four small functions that find the mean, the median and the mode of a list, and the mean of grouped data. The checker tests them on lists with an odd and an even number of values, and on a frequency table.

The idea. The mean is the total divided by the count. The median is the middle value of the sorted list, or the mean of the two middle values. The mode is the value that occurs most often. For grouped data, each class is represented by its class mark, and each class mark is counted as many times as the class frequency.

Your tasks.

1. Finish mean_of(data).
2. Finish median_of(data). Sort the list first, and look at whether the length is odd or even.
3. Finish mode_of(data). If two values tie, return the smaller one.
4. Finish grouped_mean(marks, freqs), where marks are the class marks.