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.

with open("source.txt", 'r') as source_file, open("target.txt", 'w') as target_file:
    for line in source_file:
        target_file.writelines(line)
        target_file.writelines('\n')