I once tried the npc when i was a noob,so here goes....
You start gambling,you see the beeds at the bottom part?Bronze=100,Silver=1000,Gold=1000.Put them at any numbers.There are three dice if the number total upps to what you bet you get the multiple of it(will be announced in the top left part of screen).
See the timer below the gamble part?Once it reaches 0 sec it will roll dice,if you close window and there is none gambling,the timer will reset.
The top part is very obvious,its what it say,you will get 216 time of your bet if the dices rolled are the same like what you betted.
That pretty much explains how it works.
+T if it helped
Edit:Just in case you don't understand how the small and big part works,its what is says(DUH!).If you bet small,and the dice rolls 12 and below you will win and vice-versa.
