int buzzer = 8;

void setup() {
  pinMode(buzzer, OUTPUT);
  Serial.begin(9600);
  
}

void loop() {
int i;
for (i=20; i>=0; i--){
  delay(500);
  Serial.println(i);

if(i==10 or i==20){
  digitalWrite(buzzer, HIGH);
  delay(1000);
}
else{
  digitalWrite(buzzer, LOW);
}
}

}