Given:
1, 3, 5, 7, 9, 11
Note:
1+2=3
3+2=5
5+2=7
7+2=9
9+2=11
Thus, every successive number is formed by adding 2 to the previous number.
∴ The next number can be obtained by adding 2 to the last given number, which is 11.
∴ Next number = 11 + 2 = 13