When Mendel crossed purebred purple flowering plants PP with purebred white flowering plants pp what were the flower colors of the resulting offspring?

Let us assume that both Rr and rr produce red flowers and only rr produces white flowers.

Since one parents and the offspring are white, they have rr as genotypes. In order to achieve this result, the other parent would have to haev at least one r in its genotype. Since the otehr parent is red and needs to have one r, it's genotype is Rr.

In short, the parents's genotypes are Rr for the red one and rr for the white one.