input: 
  from typing import List
  
  def calculate_rrna_percent(htseq_counts_file: str, gtf_file: str) -> float:
      """ Calculate the percentage of transcripts mapping to rRNA genes.
      >>> calculate_rrna_percent('/path/to/HTseq_counts/file', '/path/to/GTF/file')
      10.5
      """
      # Implementation of the function goes here
      pass
  
output:     
      rrna_genes = set()
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8]
                  if 'gene_biotype "rRNA"' in gene_info:
                      gene_id = gene_info.split('gene_id "')[1].split('"')[0]
                      rrna_genes.add(gene_id)
      
      total_counts = 0
      rrna_counts = 0
      with open(htseq_counts_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_id, count = line.strip().split('\t')
              count = int(count)
              total_counts += count
              if gene_id in rrna_genes:
                  rrna_counts += count
      
      return (rrna_counts / total_counts) * 100
  