There is only one mean between two numbers. It’s half their sum: (10-2)/2=8/2=4. 4 is the arithmetic mean of -2 and 10. If you wanted to find any more “means” you would take -2 with 4 and 4 with 10 and find the two means: (4-2)/2=2/2=1 and (4+10)/2=7. So the three “means” would be 1, 4 and 7. These are quartiles because they divide the range -2 to 10 into 4 groups: -2 to 1, 1 to 4, 4 to 7, and 7 to 10, each group being 3 units wide. 3 is a quarter of the range -2 to 10, which is 12 units wide.