read from source file and write to target file line by line until the end of the source file. use a function to add the numbers in the source file and write to target file the sum.

def add(a, b):
  return a + b

def main():
  with open('source.txt', 'r') as source_file:
    with open('target.txt', 'w') as target_file:
      for line in source_file:
        target_file.write(line)
        a, b = line.split()
        target_file.write(add(a, b))
        target_file.write('\n')

if __name__ == '__main__':
  main()