the number between would be the average
the average is found by adding the numbers then dividing by how many numbers there were ( in this case 2, 2/3 and 1)
so 2/3+1=1 and 2/3
divide by 2
(1 and 2/3)/2
1=3/3
1 and 2/3=3/3+2/3=5/3
then divide by 2
(5/3)/2=5/3 times 1/2=5/6
the number is 5/6
