Solve by simplifying the problem. there 18 students in mr. alvarez's art class . throughout the year, every student must pair up with every other student to complete a project. how many projects will be finished?

154, since 17+16+15...+1 = 154.
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let each of these dots represent a student. once the first student has finished a project with everyone, cover it with your finger. then, the next student has to do a project with all the others, but not the one behind him, because the one behind him already did a project with him. Am I making sense?

The answer to your question is the last one