int reading = 0; //the readings
for(int i=0; i<15; i++){ //take 15readings
int value = analogRead(lightSensor);
reading = reading + value;
}
Have you ever wondered why the language we use to program the Arduino wasn't called C=C+1? Why do you need a local variable in the loop?
int reading = 0; //the readings
for(int i=0; i<15; i++)
{ //take 15readings
reading += analogRead(lightSensor);
}