1. (O . P) -> Q 2. R -> (Q -> S) 3. O . R / P -> S ------------------------- 4. O 3 Simplification 5. R . O 3 Commutation 6. R 5 Simplification 7. O -> (P -> Q) 1 Exportation 8. P -> Q 7,4 Modus Ponens 9. Q -> S 2,6 Modus Ponens 10. P -> S 8,9 Hypothetical Syllogism