You want to take the first number if sublist #1 and add a random number between 1 and 10 to it (not specified whether this should be an integer or real), then take the first element of sublist #2 and add a random number -- in this case the range is not specified so I am assuming this is also between 1 and 10 ...and so on.
Mathematica works better when you work on entire lists rather than thinking about doing things element by element.
l = {{4,5},{7,8},{9,5},{3,2},{1,2},{13,12}}
So take all the first numbers using Part
:
l[[All,1]]
and add a list of random numbers to it using Plus
l[[All, 1]] + RandomReal[{1, 10}, Length[l]]
So in other words you simply add a list of random numbers to your list of first numbers. Depending on how to interpret your question this could be the end of it because a random number has been added to all the first numbers. However if you wanted to actually modify your original list then
l[[All, 1]] = l[[All, 1]] + RandomReal[{1, 10}, Length[l]]
We can have look at the newly modified list:
l
{{6.74801,5},{9.96048,8},{15.1219,5},{9.83344,2},{10.3313,2},{18.5355,12}}
First /@ list
orFirst[Transpose[list]]
would be the ticket... $\endgroup$Part
in the documentation, it is something you'll use a lot: reference.wolfram.com/mathematica/ref/Part.html $\endgroup$