Find the missing number?

2
7
1
9

The missing number is 1.

The numbers in the sequence are all the prime numbers less than 10. A prime number is a number that is only divisible by 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29.

In this sequence, each number is 2 more than the previous number. So, the missing number is 2 more than the last number in the sequence, which is 7. Therefore, the missing number is 1.