Answer:
Half or 0.5
Explanation:
Let the alleles for the cattle color be R and r. So,
RR = Red
Rr = Roan (incomplete dominance)
rr = white
When the breeder mates white cow (rr) with roan bull (Rr) :
   R    r
r   Rr   rr
r   Rr   rr
Half or 0.5 of the progeny has rr genotype so 0.5 of the progeny will be white in color.