You are given a code:

def main_solution(x):
    return func_20(func_20(x, 'umqtb'), func_15(x))

Can you predict the output of `main_solution("tbkfgr")` without writing any code? Please reason and put your final answer in the following json format: {"output": <your output>}, where <your output> should be the final string.
user
{"ref_input": {"x": "tbkfgr"}, "ref_output": "t\u00abutb\u00bbm\u00abkbq\u00bbf\u00abtkg\u00bbb\u00abrf\u00bb\u00abg\u00bb\u00abr\u00bb", "ref_code": "def main_solution(x):\n    return func_20(func_20(x, 'umqtb'), func_15(x))", "funcname": "main_solution"}
rule