esp32: Doing some math in c++ results in inf

Question

I'm currently working on a project and i need to calculate time between to presses of a button and do some calculations on it and then show it on a display. currently I'm only printing it and it only prints inf and not a number. here is my code:

const int ledPin = 5;
const int btnPin = 33;
unsigned long start_time =1;
unsigned long time_passed;
double current_speed;
void setup() {
  Serial.begin(115200);  
  pinMode(ledPin, OUTPUT);
  pinMode(btnPin, INPUT_PULLDOWN);
}

void loop() {
         bool buttonState = digitalRead(btnPin);
        if (buttonState==1){ 
          time_passed = (millis() - start_time); //calculating time between to presses
            if (time_passed > 25){ // checking if button is actually pressed and not hold
            Serial.println(0.35/(time_passed/1000)*3.6); // this outputs "inf"
            }
          start_time = millis(); // resetting time
          }
}