1. P ∨ Q 2. (Q ∙ ~R) → S 3. R → P 4. ~P ∴ S --------------------------- 5. ~R 3,4 Modus Tollens 6. Q 1,4 Disjunctive Syllogism 7. Q ∙ ~R 6,5 Conjunction 8. S 2,7 Modus Ponens