Thanks, I have just fathomed it out! First column of numbers increase by 2 e.g 5 7 9 11 13 15 17 19 21. Second column of numbers increase by 1 e.g 4 5 6 7 8 9 10 11 12. Match the numbers up and 12 equates to 21!
The silly thing about number puzzles is that you can often have two, three or even more different but perfectly correct solutions!
I think I'll stick to crosswords :-)