v = 340m/s = 0.034cm/us (centimeter/microseconds)
time = distance/speed
time = 40/0.034
time = 1176 microseconds

distance = time x speed of sound/2
distance = time x 0.034/2

#define ECHOpin 5 // it defines the ECHO pin of the sensor to pin 5 of Arduino
#define TRIGpin 6 
// we have defined the variable
long duration; // variable for the duration of sound wave travel
int distance; // variable for the distance measurement
void setup() 
{
  pinMode(TRIGpin, OUTPUT); // It sets the ECHO pin as OUTPUT
  pinMode(ECHOpin, INPUT); // It sets the TRIG pin as INPUT
  Serial.begin(9600); // // Serial Communication at the rate of 9600 bps
  Serial.println("Test of the Ultrasonic Sensor HC-SR04"); // It will appear on Serial Monitor
  Serial.println("with the Arduino UNO R3 board");
}
void loop() 
{
  // It first sets the TRIG pin at LOW for 2 microseconds
  digitalWrite(TRIGpin, LOW);
  delayMicroseconds(4);
  // It now sets TRIG pin at HIGH for 15 microseconds
  digitalWrite(TRIGpin, HIGH);
  delayMicroseconds(15);
  digitalWrite(TRIGpin, LOW);
  // It will read the ECHO pin and will return the time 
  duration = pulseIn(ECHOpin, HIGH);
  // distance formula
  distance = duration*(0.034/2); // (speed in microseconds)
  // Speed of sound wave (340 m/s)divided by 2 (forward and backward bounce)
  // To display the distance on Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm"); //specified unit of distance
}