What was the percentage change in the free cash flow between 2018 and 2019?
{"Cash flows provided by operating activities": {"2019": 115.3, "2018": 66.8}, "Payments for property, plant and equipment": {"2019": -90.6, "2018": -62.8}, "Free cash flow": {"2019": 24.7, "2018": 4.0}}
free_cash_flow_2019 = df["Free cash flow"]["2019"]
free_cash_flow_2018 = df["Free cash flow"]["2018"]
answer = ( free_cash_flow_2019 - free_cash_flow_2018 ) / free_cash_flow_2018 * 100.0