If we look at mark between Empty and 1/4, we see that it divided in two equal parts , or
1/4 divided by 2.
1/4 : 2 = 1/4 * 1/2 = 1/8
That means that to 1/2 is added 1/8.
1/2 + 1/8 = 4/8 +1/8 = 5/8
Answer is 5/8.
Or you can count by 1/8 up to arrow 1/8, 2/8,...5/8.
And you will get 5/8 also.