The next number is 26. The easiest way to do these kinds of problems without using the arithmetic equation and stuff is basically find the pattern between the numbers. 6 + 5 = 11. 11 + 5 = 16. 16 + 5 = 21. 21 + ? = ? From this, we see that the number 5 is being added so the next number is 21 + 5 which is 26. I hope this helps :-)