应交水费y与月用水量x相关：当x不超过15时，y=4x/3超过后，y=2.5x−10.5。实现水费的计算，要求定义和调用函数fun(x)计算水费，其中x的类型是double函数类型是double
double fun(double x);
y = fun(x);
double fun(double x)
return result;
double fun(double x);
y = fun(x);
double fun(double x)
return result;
double fun(double x);
y = fun(x);
double fun(double x)
return result;
double fun(double x);
y = fun(x);
double fun(double x)
return result;