Incorrect Answer.

		1 p q r	premise
		2 q r	assumption
		3 q	e1 2
		4 p	e 1, 2
		5 p q	i 2-4

• It employs the right global stragety: use -introduction to show the implication p q. However, this requires that we assume p in line 2. If -introduction is the last proof rule we intent to use, and if our proof begins as in the first two lines above, there is no way that this can ever type check! Moreover, the formula in line 4, the conclusion of the box, would have to be q, and not p, for -introduction to work.
• Notice also that the application of -elimination in line 4 does not type check. Although its arguments type check well, the rule's conclusion has to be q r, the conclusion of the cited implication in line 1, and not p.