What should be added to the difference of 1 and 1/2 and 2/6 to get 1 and 2/3?

Rewrite 1/2 to have a common denominator with 2/6

1/2 x 3 = 3/6

Now you have 1 and 3/6 - 2/6 = 1 and 1/6

Now rewrite 2/3 to have a common denominator with 1/6: 2/3 x 2 = 4/6

Subtract 1/6 from 4/6:

4/6 - 1/6 = 3/6 = 1/2

You will need to add 1/2