Paddy wants a job, but the foreman won't hire him until he passes a little maths test.   
 Here is your first question, the foreman said.  "Without using numbers, represent the number 9." 
"Without numbers?"  Paddy says? "Dat's easy." And proceeds to draw three trees.

"What's this?" the boss asks. 
"Have you no brain?  Tree and tree plus tree makes 9" says Paddy
"Fair enough," says the boss.  "Here's your second question.  Use the same rules, but this time the number is 99." 
Paddy stares into space for a while, then picks up the picture that he has just drawn and makes a smudge on each tree... "Ere ye go."

The boss scratches his head and says, "How on earth do you get that to represent 99?" 
"Each of them trees is dirty now.  So, it's dirty tree, and dirty tree, plus dirty tree. Dat makes 99." 
The boss is getting worried that he's going to actually have to hire Paddy, so he says, "All right, last question.  Same rules again, but represent the number 100." 
Paddy stares into space some more, then he picks up the picture again and makes a little mark at the base of each tree and says, "Ere ye go.  One hundred."

The boss looks at the attempt.  "You must be nuts if you think that represents a hundred!" 
Paddy leans forward and points to the marks at the base of each tree and whispers, "A little dog came along and pooped by each tree. 
So now you got dirty tree and a turd, dirty tree and a turd, and dirty tree and a turd, which makes ONE HUNDRED!" 
Paddy is the new supervisor.