input: 
  from typing import List
  
  def percent_rRNA_mapping(htseq_counts: str, gtf_file: str) -> float:
      """ Calculate the percent of transcripts mapping to rRNA given an HTseq_counts file and a GTF file.
      >>> percent_rRNA_mapping('C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt', 'Mus_musculus.GRCm38.84.gtf')
      12.345
      """
  
output:     
      rRNA_count = 0
      total_count = 0
  
      with open(htseq_counts, 'r') as htseq_file:
          for line in htseq_file:
              if not line.startswith("__"):
                  total_count += 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].strip('"')
                  with open(htseq_counts, 'r') as htseq_file:
                      for htseq_line in htseq_file:
                          if gene_id in htseq_line:
                              rRNA_count += int(htseq_line.split()[1])
                              break
  
      return (rRNA_count / total_count) * 100
  