In column A I have a student Id#. In column B I have a semster 1 and/or 2. So column A should have the same student ID# twice (once for semster 1 and once for semster 2). In column C is the Homeroom teacher name (or code). Is there a way to find if a student in column A has two different Homeroom teachers in column C? They should look like this (except student 232 has a different homeroom teacher for semester 2). Is there a formula for finding these students?
A        B     C
222      1    704
222      2    704
423      1    708
423      2    708
232      1    801
232      2    805

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Answer from Richard Rost:

Sure. You could use the COUNTIF function to determine how many homeroom teachers each student has.

You can use the SUMIF and COUNTIF functions to add up or count values based on specific criteria, respectively.

Here is a free tutorial that explains how to use both:

http://www.599cd.com/tips/excel/sumif-countif?key=BlFAlEx