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_file(source, target):
  with open(source) as source_file, open(target, 'w') as target_file:
    for line in source_file:
      target_file.write(str(sum(map(int, line.split()))) + '\n')

add_file('source.txt', 'target.txt')