Theory
An average is a number that represents a typical value in a data set. The three GCSE averages are mean, median and mode.
The mean is found by adding all the values and dividing by how many values there are. It uses every number, so one very large or very small value can pull the mean away from the middle.
The median is the middle value after the data has been put in order. If there are two middle values, add them and divide by 2.
The mode is the value that appears most often. A data set can have one mode, more than one mode, or no mode.
The range is not an average. It measures spread: range = largest value - smallest value. A large range means the data is more spread out.
Foundation students should slow down at the ordering step. Many lost marks come from finding the median before putting the numbers in order.