Earlier I provided an "answer" as a hint (in the form of a question). It was deleted as being more appropriate as a comment ("request for clarification") it was not a request for clarification. It was a hint in the form of a question.
"Hint: How many total A's were there out of how many total grades?"
The point is that there were total A's out of
total grades. Hence there were
grades that were not A's. If these grades were maximally spread around among the
students, there would have been
students who got a non A. (There may have been less such students, as some students could have gotten more than one non A.) But in any case that would mean that there are least
students who got all A's.
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Earlier I provided an "answer" as a hint (in the form of a question). It was deleted as being more appropriate as a comment ("request for clarification") it was not a request for clarification. It was a hint in the form of a question.
"Hint: How many total A's were there out of how many total grades?"
The point is that there were total A's out of
total grades. Hence there were
grades that were not A's. If these grades were maximally spread around among the
students, there would have been
students who got a non A. (There may have been less such students, as some students could have gotten more than one non A.) But in any case that would mean that there are least
students who got all A's.
Let be the number of students that got all As.
We use the inclusion-exclusion formula,
to get
To find a bound, we need to bound the intersections: using we can rearrange to obtain
Plugging that (and analogous expressions) back into the equality for
gives
Thus , because
.