The number of miles john ran are 10,11,17,20,16,14,14 and 23. Which statistical measure should john use to determine how much he improved. Find the measure

Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 or more marbles, the students have a party. right now, the the jar has 24 marbles. how could mrs. morton reward the class in order for the students to have a party?