When mendel crossed purebred purple flowering plants (pp) with purebred white flowering plants (pp), what were the flower colors of the resulting offspring?

The answer is 100 percent purple

Let P be dominant allele for purple flowers and p recessive allele for white flowers. So purebred purple flower has two P alleles and its genotype is PP and purebread white flower has two p alleles and its genotype is pp. Since P is dominant over p, when found together like in heterozygous plants (Pp), the flowers will express dominant allele and will be purple.
So, a cross between purebred purple flowering plants (PP) with purebred white flowering plants (pp) will have 100% of the offspring with genotype Pp and purple flower color.

hope that help