An accountant, a lawyer, and an engineer are debating the relative merits of having a wife or a mistress.
The accountant says it's better to have a wife. The arrangement is dependable. It's stable and predictable.You can count on it.
The lawyer says it's better to have a mistress. It's exotic. It's exciting. And if you don't like it, you just walk out.
The engineer says, you guys don't understand the situation. You need both a wife and a mistress. You can let your wife think you're with your mistress, and let your mistress think you're with your wife. Then you can sneak off and get some work done.
The mathematician busts out his calculator, punches a few figures in and answers, "4"!
The engineer busts out his compass, protractor, and paper, draws a few things and answers, "4"!
The accountant quietly gets up, walks over to the door and closes it, returns to his seat and answers, "What do you need it to be?"