1970's (New Math)
A farmer exchanges a set P of potatoes with a set M of money. The cardinality
of the set M is equal to $10 and each element of M is worth $1. Draw 10 big
dots representing the elements of M.
The set C of production costs is comprised of 2 big dots less than the set M.
Represent C as a subset of M and give the answer to the question: What is the
cardinality of the set of profits? (Draw everything in red).
-- from: The evolution of mathematics education during the last 30 years.