Question: Decide which of the following two cond-expressions is legal: 

(cond                   (cond
  [(< n 10) 20]           [(< n 10) 20]
  [(> n 20) 0]            [(and (> n 20) (<= n 30))]
  [else 1])               [else 1])
  
Answer:  The cond-expression on the left is legal.  
         The cond-expression on the right contains a clause with a question but with no
         answer.
 

 
Question: Why is the following illegal? 

(cond [(< n 10) 20]
      [* 10 n]
      [else 555]) ; 

Answer: Firstly, the clause [* 10 n] contains three expressions, instead of two. Secondly
        the clause must contain a question and answer which must equate to either true or
        false.