20 heterozygous purple flowering pea plants are crossed. if purple is dominant over white, what are the expected phenotypic results?

Let's call the colour alleles C for dominant purple, and c for recessive white.

We have crossed Cc with Cc.  So if we put that in a Punnett square, we'd have a CC, two Cc and a cc.  That would give us a 3:1 ratio of purple to white, so you'd expect 15 purple and 5 white flowers. :)

That would be a working theory.