Răspuns :
13
113, 213, 313,....,913 => 9 nr
1013, 1113, 1213,....., 1913 => 9 nr
2013, 2113, 2213,....., 2913 => 9 nr
...............................................................
9013, 9113, 9213,....., 9913 => 9 nr
(9 randuri a cate 9 nr => 9×9=81 nr)
81+9+1=91 de numere mai mici decat 10000 care se termina in 13.
Raspuns: 91 :P