21
13
9
11
Answer is Right!
Answer is Wrong!
The missing number is 11.
The numbers in the sequence are all the prime numbers less than 20, except for 13. 11 is the next prime number after 13.
A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number.