int number;
int x;

void setup()
{
  Serial.begin(9600);
}

void loop()
{
 for(number = 1; number < 21; number++)
 {
  x = number % 5;
  Serial.print(number);
  Serial.print(" modulo ");
  Serial.println(x);
  delay(1000);
 }
}

// SERIAL OUTPUT

number 1 modulo 1
number 2 modulo 2
number 3 modulo 3
number 4 modulo 4
number 5 modulo 0
number 6 modulo 1
number 7 modulo 2
number 8 modulo 3
number 9 modulo 4
number 10 modulo 0
number 11 modulo 1
number 12 modulo 2
number 13 modulo 3
number 14 modulo 4
number 15 modulo 0
number 16 modulo 1
number 17 modulo 2
number 18 modulo 3
number 19 modulo 4
number 20 modulo 0