21Hint for 9th one: According to property of reflection a ray originating from B will retrace the path. Therefore by symmetry angle made by the ray originating from A with AB and that of B with AB will be same, say θ. Again by symmetry the ray will incident at exactly the midpoint of DC. If u draw you will see that the path of light makes three congruent triangles. 2/√13 is the correct answer.
