.
In a survey of 225 students, 100 were taking mathematics and 70 were taking psychology.
What is the least number of students that could be taking both courses
~~~~~~~~~~~~~~~~~~~~~~~~~
Answer. 0 (zero, ZERO).
Solution
We are given the set consisting of 225 elememts.
And two its subsets: one of 100 elements and the other of 70 elements.
We don't know if these subsets have empty or non-empty intersection.
The problem asks to estimate the minimal number of elements in the intersection.
The answer is 0 (zero, ZERO), since the two subsets may have the empty intersection:
there is no obstacles or restrictions; nothing prohibits it.