What must be added to 1/6 to make 1/4?

To find what must be added to 1/6 to make 1/4, we subtract 1/6 from 1/4:

1/4 - 1/6 = (3/12) - (2/12) = 1/12

Therefore, 1/12 must be added to 1/6 to make 1/4.