input: 
  from typing import List
  
  def calculate_rRNA_percentage(htseq_counts: str, gtf_file: str) -> float:
      """
      Calculate the percentage of transcripts mapping to rRNA in the given HTseq_counts file using the provided GTF file.
      
      >>> calculate_rRNA_percentage("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")
      18.123
      """
  
output:     
      rRNA_counts = 0
      total_counts = 0
      
      with open(htseq_counts, 'r') as htseq_file:
          for line in htseq_file:
              if not line.startswith("__"):
                  total_counts += int(line.split()[1])
      
      with open(gtf_file, 'r') as gtf_file:
          for line in gtf_file:
              if "rRNA" in line:
                  gene_id = line.split('\t')[8].split(';')[0].split('"')[1]
                  with open(htseq_counts, 'r') as htseq_file:
                      for htseq_line in htseq_file:
                          if gene_id in htseq_line:
                              rRNA_counts += int(htseq_line.split()[1])
      
      return (rRNA_counts / total_counts) * 100
  