1. p -> (q -> r) 2. ~ (q & ~r) -> s 3. s -> t 4. t <--> (q & ~s) Therefore: p <--> s ---------------------------------------- 5. ( ~q v ~~r) -> s 2 DeMorgan's Theorem 6. ( ~q v r) -> s 5 Double Negation 7. (q -> r ) -> s 6 Material Implication 8. p -> s 1,7 Hypothetical Syllogism 9. [t -> (q & ~s)] & [(q & ~s) -> t] 4 Material Equivalence 10. t -> (q & ~s) 9 Simplification 11. ~t v (q & ~s) 10 Material Implication 12. (~t v q) & (~t v ~s) 11 Distribution 13. (~t v ~s) & (~t v q) 12 Commutation 14. ~t v ~s 13 Simplification 15. t -> ~s 14 Material Implication 16. s -> ~s 3,15 Hypothetical Syllogism 17. ~s v ~s 16 Material Implication 18. ~s 17 Tautology 19. ~s v p 18 Addition 20. s -> p 19 Material Implication 21. (p -> s) & (s -> p) 8,20 Conjunction 22. p <--> s 21 Material Equivalence