On a ruler with markings from 0 to 30, where each major marking is a whole number and there are 10 small divisions between each whole number, points A through K are located as follows: A=20.0, B=20.5,

Position of A $= 20.0$
Position of B $= 20.5$

Distance $= \text{Position of B} - \text{Position of A}$
Distance $= 20.5 - 20.0$
Distance $= 0.5$

The distance from A to B is $\boxed{\text{0.5}}$.

5) How far is it from D to G?

Position of D $= 21.5$
Position of G $= 23.5$

Distance $= \text{Position of G} - \text{Position of D}$
Distance $= 23.5 - 21.5$
Distance $= 2.0$

The distance from D to G is $\boxed{\text{2.0}}$.

6) How far is it from G to K?

Position of G $= 23.5$
Position of K $= 27.0$

Distance $= \text{Position of K} - \text{Position of G}$
Distance $= 27.0 - 23.5$
Distance $= 3.5$

The distance from G to K is $\boxed{\text{3.5}}$.