Am I making harder then it is based on this question: See my answer below.
F2 population of a plant species (which is a diploid) resulted from a cross between a plant with red flowers and a plant with white flowers. The flower color is controlled by a single gene; each of the parents is homozygous for the gene; the red allele is dominant over the white allele. When you scored 20 F2 plants, what is the probability to have the number of plants with white flowers is 2 or fewer?
probability = 2/20 + 1/20 + 0/20 = 0.15