input: 
  from typing import List
  
  def calculate_percent_rRNA(htseq_counts_file: str, gtf_file: str) -> float:
      """ Calculate the percent of transcripts mapping to rRNA.
      >>> calculate_percent_rRNA('C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt', '/Users/malwina/Documents/CTR-Data/genomes/Mus_musculus/mm10/Mus_musculus.GRCm38.84.gtf')
      3.45
      """
  
output:     
      rRNA_count = 0
      total_count = 0
  
      with open(htseq_counts_file, 'r') as htseq_file:
          for line in htseq_file:
              gene_id, count = line.strip().split('\t')
              total_count += int(count)
              if gene_id.startswith('rRNA'):
                  rRNA_count += int(count)
  
      percent_rRNA = (rRNA_count / total_count) * 100
      return round(percent_rRNA, 2)
  