Answer: 25% of the offsprings will be heterozygous for both traits
Explanation: A cross between BbFf and bbff will yield 16 offsprings with four different possible genotypes: BbFf, Bbff, bbFf, and bbff. Only BbFf is the genotype that is heterozygous for both traits, and there are four offsprings with BbFf genotype.
Therefore the percentage of the offsprings that are heterozygous for both traits is 4/16 x 100 = 25%
See the punnett square for the details of the dibybrid cross