One assumes that purple is dominant and white is recessive.
So, Pw, would be the simple Mendelian genotype. The cross is called a test cross. You use a recessive expressed plant for this becaus the recessive must be homozygous to express the white color.
Pw X ww
Now, if the purple flower offspring has the allele for white this test cross will show you that 1/2 of the offspring will be white. Just use your Punnet square to do these simple crosses.
