According to medical tests, when a person is still, a lot of heat is lost from the head. Generally, when the temperature is 15°C, the heat lost from the head accounts for 30% of the total heat lost by the human body; when the temperature is 4°C, from The heat lost from the head accounts for (3/5) of the total heat lost by the human body; when the temperature is minus 15°C, the heat lost from the head accounts for (3/4) of the total heat lost by the human body; when the temperature is minus 20°C, from If the heat lost from the head accounts for 0.8 of the total heat lost by the human body, what is the temperature at which the most heat is lost from the head?
据医学测试，人静止不动时，从头部散失的热量很多，一般情况下，气温为15℃时，从头部散失的热量占人体散失总热量的30%；气温为4℃时，从头部散失的热量占人体散失总热量的(3/5)；气温为零下15℃时，从头部散失的热量占人体散失总热量的(3/4)；气温为零下20℃时，从头部散失的热量占人体散失总热量的0.8，则从头部散失热量最多时的气温是多少．
<result>20</result>