Replace lines from one .txt file to another

[Tumppi]

I am a beginner to do any coding, but still i'll try to do something..  

My problem:

i need copy some lines Start.txt to end.txt, only what i know is line numbers, marks/numbers inside the line doest not same all the time, line numbers are. So i cant use .replace method?

Example

Start.txt

       1111111111
       2222222222
       3333333333
       4444444444  <------ This line
       5555555555  <------ And this line

End.txt

       888888888
       999999999
       AAAAAAAAA
       BBBBBBBBB
                 <----- to here
                 <----- to here

i was find some enumerate example before, and try to adjust it, but too hard from beginner.
Something like this what i find:

       with open('start.txt') as fin, open('end.txt', 'w') as fout:
           for i, item in enumerate(fin, 1):
               if i == 7: 
                   item = "string\n" 
               fout.write(item)

This code add only "string" word to line which number i gave...


Thanks!