25%
Following a cross between Ttbb and ttBb, the resulting offspring would be:
TtbB , TtbB , Ttbb , Ttbb , ttbB , ttbB , ttbb , ttbb , TtbB , TtbB , Ttbb , Ttbb , ttbB , ttbB , ttbb , ttbb
To have a clearer idea, see the attached image.
Of the 16 possible combinations from this dihybrid cross, there will be:
4 tall plants with blue flowers
4 tall plants with white flowers
4 short plants with blue flowers
4 short plants with white flowers
There are 4 possible genotypic recombinations that would yield short plants with white flowers out of a possible 16, so the chance that the offspring will be short with white flowers would be 4/16 which is 1/4 or 25%