This problem appears in a middle-school mathematics textbook: Together Dan and Diane have $20. Together Diane and Donna have $15. How much do the three of them have in total? (Transition Mathematics, Second Edition, Scott Foresman Addison Wesley, 1998

If x, y and z represent the money held by Dan, Diane and Donna, then y = 15-z and x = 20-y = 20-(15-z) =5 + z. We can let z take on any value from 0 to 15 without any of the three amounts being negative, since presumably middle-schoolers are too young to assume debt. Then the total capital held by the three is x + y + z = (5 + z) + (15 - z) + z = 20 + z. So their combined holdings can range anywhere from $20 (Donna is broke) to $35 (Donna is flush).