Observe the characteristics of the numbers given and write the numbers underlined below.~(adapted from $$2006$$ Math kangaroo Problems, Level $$1-2$$, Question \#$$8$$)  $$3$$~ ~ $$4$$~ ~ $$7$$~ ~ $$11$$~ ~$18$~\uline{~~~~~~~~~~}~$47$~ ~$76$~ ~$123$~ $$\cdots$$ 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Arithmetic Sequences->Concepts of Arithmetic Sequences
The sum of the first digit and the second digit is the third digit, and so on. The horizontal line is $11+18=29$. 