int red_pin = 13;
int ylw_pin = 9;
int grn_pin = 6;

unsigned long st; // started time for system
unsigned long ct;  // current_time

bool red_state;
bool ylw_state;
bool grn_state;

void setup() {
  pinMode(red_pin, OUTPUT);
  pinMode(ylw_pin, OUTPUT);
  pinMode(grn_pin, OUTPUT);
}

void loop() {
  ct = millis();

  if (ct - st > 15000) {
    st = ct;
  }

  if (ct - st < 7000) {
    red_state = HIGH;
  }
  else {
    red_state = LOW;
  }

  if ((ct-st>5000 && ct-st<7000) || (ct-st>13000)) {
    ylw_state = HIGH;
  }
  else {
    ylw_state = LOW;
  }

  // write the code here for the green
  


  digitalWrite(red_pin, red_state);
  digitalWrite(ylw_pin, ylw_state);
  digitalWrite(grn_pin, grn_state);
}