How can this English sentence be translated into a logical expression? ( Translating " unless") 
You cannot ride the roller coaster if you are under 4 feet tall unless you are older than 16 years old.

Let:


*

*$P$ stands for "you can ride the roller coaster"

*$Q$ stands for "you are under 4 feet tall"

*$R$ stands for "you are older than 16 years old"
Is this logical expression correctly translated?
$$P \rightarrow (Q \wedge R)$$

(1) 'Unless' statements:
There are some known strategies to transform 'unless' clauses into conditional statements. The most common one seems to be directly translate them using 'if not':


*

*I'm not coming to the party unless Sylvia comes.

*I wouldn't eat that food unless I was really hungry.
The examples above can be respectively translated as follows:


*

*If Sylvia is not coming to the party, neither am I.

*If am not really hungry I wouldn't eat that food.
Alternatively, we can use their (reverse) contrapositive forms:


*

*I am coming to the party if Sylvia is.

*I would eat that food If was really hungry.
(2) Your Answer:
Consider the English sentence

You cannot ride the roller coaster if you are under 4 feet tall unless you are older than 16 years old.

Following the above reasoning we have:

If you are not older than 16 years, then you cannot ride the roller coaster if you are under 4 feet tall.

which is the same as:

If you are not older than 16 years, then if you are under 4 feet tall you cannot ride the roller coaster.

Now let: 


*

*$P$ stand for 'you can ride the roller coaster'

*$Q$ stand for 'you are under 4 feet tall'

*$R$ stand for 'you are older than 16 years old'


The answer you are looking for is
$$ \neg R \to (Q \to \neg P).$$
