51 84 99 8 55 13 12 34 this is a regular group of numbers. What should I fill in the question mark?

51 84 99 8 55 13 12 34 this is a regular group of numbers. What should I fill in the question mark?

51 84 99 5+4=9 1+8=9 99
13 12 34 1+2=3 3+1=4 34
The answer is 47