We have that DNA is transcribed into mRNA in a one to one fashion, namely one nucleotide of DNA is transcribed to 1 of mRNA. By the genetic code, we have that 3 nucleotides correspond to 1 amino acid. Hence, we have that if in a minute a polymerase transcribes 60 nucleotides, these correspond to 60/3 aminoacids. Hence, the rate has to be at most 20 aminoacids per minute.