If the two genes are linked and 10 m. U. apart, the recombination frequency between them is 10%. This means that 10% of the gametes produced by the F1 heterozygotes will be recombinant (ab/ab or AB/AB). The remaining 90% of the gametes will be non-recombinant (ab/AB or AB/ab).
In the test cross, each of the F1 heterozygotes will produce two types of gametes in equal proportions - those with the original allele combination (ab) and those with the recombinant allele combination (ab or AB). Therefore, 50% of the test cross progeny will be ab/ab and 50% will be ab/AB or AB/ab.
Since the genes are linked and the recombination frequency is 10%, the proportion of ab/AB or AB/ab progeny will be higher than expected if the genes were unlinked (where the proportion of non-parental types would be 50%). Therefore, the proportion of ab/ab progeny will be lower than 50%, but the exact proportion depends on the degree of linkage and the distance between the genes.