In a bacterial conjugation experiment, a researcher wanted to study the transfer of a plasmid containing a gene for resistance to streptomycin from a donor strain to a recipient strain. The donor strain was resistant to streptomycin and carried the plasmid, while the recipient strain was sensitive to streptomycin and lacked the plasmid.

The researcher interrupted the mating at different time intervals and plated the conjugation mixture on a selective medium containing streptomycin to identify the transconjugants. The researcher obtained the following data:

Time (minutes)   Number of transconjugant colonies
     0                              0
   10                            20
   20                           100
   30                           300
   40                           400

What is the transfer rate between 20 and 30 minutes?