On 22, note CE and AF are medians of $\triangle ABC$. What do you know about the medians of a triangle?

For 24, recall that the ratio of the areas of two triangles with the same height is the ratio of their bases. So, what is $[CBD]/[ABC]$? Can you find other pairs of triangles that have the same height?

On 29,  since $ED$ and $GF$ are parallel to $BC$, $\triangle ADE \sim \triangle AFG \sim \triangle ABC$. Remember also that the ratio of the areas of similar triangles is the square of the ratio of their sides.