'Thirty' in the Bible

The men of Ai put to death about thirty-six of them, driving them from before the town as far as the stoneworks, and overcoming them on the way down: and the hearts of the people became like water.

So Joshua and the fighting-men got ready to go up against Ai; and Joshua took thirty thousand men of war, and sent them out by night.

The king of Tirzah, one; all the kings together were thirty-one.