@ShanthanK and @Lakshman Patel RJIT
I am interpreting the sentence as (you cannot ride the roller coaster if you are under 4 feet tall) unless (you are older than 16 years old)
which gives (~q if r) unless (s)
which gives (r→ ~q) unless (s)
which gives (~s)→ (r→ ~q) (using p→ q: q unless ~p)
which gives (~s ^ r )→ ~q (using p→ (q→ r) = (p ^ q) → r )
which is the same as the ans given in Rosen book, i.e (r ^ ~s)→ ~q
NOTE: The only trick( that I feel) was in the last step
i.e, p→(q→r) = (p ^ q)→r
This is also called as Exportation Law . ( You can google it ;) )
If someone wants the proof of above, I am giving it here-
LHS: p→ (q→ r) = p→ (~q v r)
=~p v (~q v r)
=(~p v~q) v r
=~(p ^ q) v r
=(p ^ q)→ r : RHS
Correct me if I am wrong. Happy Learning guys.