If all the F1 generation from true-breeding parents are purple than the genotype would be heterozygous i.e., Pp and Pp. Here P indicates the dominant allele which produces purple flower and p indicates the recessive allele of white flower.
So if we will cross the F 1 generation than the genotype would be PP, Pp, Pp, pp. That means 3 out of 4 offspring would be purple because three genotypes out of four have dominant purple allele(P).
So if out of 4 flower 3 is purple than according to calculation out of 500 flower 375 offspring would be purple(3/4*500= 375). Therefore the correct answer is 375 purple flowers should be produced.
Because they are similar, we can say that segment AC corresponds to segment DF, and segment AB corresponds to segment ED. So, we can set up a proportion.