In a village, there lived a kindly baker who sold many items (cakes, biscuits, buns, bread loaves, croissants, etc) in his small shop. He made enough profits for running his family.
Once, he made a dozen loaves of a special bread each of which cost him 2 U (U is some unit of currency) to make. He usually sold it for 3 U each, when the buyer was rich; but he kindly sold it for 1 U each, when he knew that the buyer was poor. After closing-time, he found that he was sold-out and had made a profit of 3 U on the bread alone.
Now Dear Reader, something for you to think over :
How is this situation possible ?
How many rich buyers bought the special bread ?
Depending on how you usually think, you may treat it as one unknown or 2 unknowns.
If you think there is one unknown, then there are actually 2 unknowns.
If you think there are two unknowns, then there are actually 3 unknowns.
Basically, there is one EXTRA unknown.