def factorial(n)
  """Calculate factorial recursively"""
  if n <= 1
    return 1
  else
    return n * factorial(n - 1)
  end
end
def is_prime(num)
  if num < 2
    return false
  end
  for i in 2...Integer(num ** 0.5) + 1
    if num % i == 0
      return false
    end
  end
  return true
end
print("Factorial of 5: #{factorial(5)}", "\n")
for n in [2, 3, 4, 17, 21]
  print("#{n} is prime: #{is_prime(n)}", "\n")
end