Bartley MacDermott wants a job, but the boss won’t hire him until he passes a simple math test. The boss says:
Bartley say:
Bartley proceeds to draw three trees:

The boss asks:
To which Bartley replies:
The boss responds:
Use the same rules, but this time the number is 99.”
Bartley stares into space for a while, then picks up the picture that he has just drawn and makes a smudge on each tree.. “There ye go.”

The boss scratches his head and says:
“How on earth do you get that to represent 99?”
To which Bartley replies:
“Each of the 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 actually going to have to hire Bartley,
so he says:
“All right, last question. Same rules again,
but represent the number 100.”
Bartley stares into space a bit,
then he picks up the picture and makes a little mark at the base of each tree and says, “Here ye go. One hundred.”

The boss looks at the picture:
“You must be crazy if you think that represents a hundred!”
Bartley leans forward and points to the marks at the base of each tree
and says:
“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 100.”
Barley is now the new supervisor!