A twist on error-analysis, these problems have already been "worked out by other students." YOUR students have the privilege of "grading" each paper, identifying errors, and providing constructive feedback. Some problems are correct, but other's aren't... can your students tell the difference?
Additionally, each problem has a QR code which can be scanned for hints. For example, correct problems' QR codes state a text message that the problem is already correct; incorrect problems' QR codes d