double time = Convert.Time(Double.parseDouble(txt_Size.getText().toString()), size_type,Double.parseDouble(txt_Speed.getText().toString()),speed_type);

static double Time(double size, String Size_type, double speed, String speed_type) {

    if (Size_type.equals(speed_type) )
    {
        return size / speed;
    }
    else if (Size_type.equals("KB") || speed_type.equals("KB"))
    {
        return to_Kb(size, Size_type) / to_Kb(speed, speed_type);
    }
    else if (Size_type.equals("MB") || speed_type.equals("MB"))
    {
        return to_Mb(size, Size_type) / to_Mb(speed, speed_type);
    }
    else if (Size_type.equals("GB") || speed_type.equals("GB"))
    {
        return to_Gb(size, Size_type) / to_Gb(speed, speed_type);
    }
    else
    {
        return -1;
    }
}