The answer is 75%
Let's assign these genotypes to the flowers:
Purple- P (dominant)
White- pp (recessive)
Two heterozygous purple flowers are crossed:
Pp X Pp
The resulting offspring of the cross would be:
PP- 25% homozygous purple flower
Pp- 50% heterozygous purple flower
pp- 25% homozygous white flower
PP, Pp- are purple flowers
25%+50%= 75%