An elderly man wants a job, but the foreman won’t hire him until he passes a little maths test:
“Here is your first question.” The foreman says. “Without using numbers, represent the number nine?”
“Without numbers?” The old man says. “That’s easy.”

And he proceeds to draw three trees.

“What’s this?” The boss asks.
“Have you no brain? Tree plus tree plus tree makes nine.” Says the old man.
“Fair enough.” Says the boss.
“Here’s your second question. Use the same rules, but this time the number is 99?”

The man stares into space for a while, then picks up the picture that he has just drawn and makes a smudge on each tree and hands it back.

The boss scratches his head and says.
“How on earth do you get that to represent 99?”
“Each of the trees is dirty now. So, it’s a dirty tree plus a dirty tree plus a dirty tree. That makes 99.”
“All right, last question. Same rules again, but represent the number 100?”

The old man stares into space again; he then picks up the picture and makes a little mark at the base of each tree and hands it back.
The boss looks at the man’s picture.
“You must be nuts if you think that represents a hundred?”
The old man 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, plus dirty tree and a turd, plus dirty tree and a turd, which makes 100.”