what probability do we have of obtaining a purple flower (Rr), when we combine two blue flowers (rr) in an incomplete Dominance relationship?

Answer: none

Explanation:

If we have that the color characteristic is regulated by the dominant gene R, by crossing two blue flowers with the recessive gene r, no violet flower will be obtained, since the rule of incomplete dominance describes that phenotypes can be obtained intermediates of homozygous parents, however in the case, we only have 2 parents with the same recessive conditions, which do not contribute to generate the violet, to produce one of the parents should have a gene that expresses the color red