Exam marking related improvements

Doc/Sd1/Exam/2021/Winter/exam.xml

@@ -307,8 +307,9 @@ public static byte getAverage(byte a, final byte b, final byte c) {
 
             <para>Explain both points in the answer box below.</para>
 
-            <para>In addition provide a solution by modifying the above code.
-            You may present your code in the answer box below as well.</para>
+            <para>In addition provide a solution by modifying the above
+            implementation without changing the method's signature. You may
+            present your code in the answer box below as well.</para>
 
             <para>Alternatively correct the implementation of
             <package>de.hdm_stuttgart.mi.sd1.task4_no_unit_test</package>.<classname>Mathextend</classname>
@@ -473,6 +474,12 @@ System.out.println("Result: " +
 
 // Note: final short sum = a + b + c does not even compile as being explained at
 // https://freedocs.mi.hdm-stuttgart.de/sd1_sect_arithmeticOperators.html#sd1_explainNoByteByteOperator</programlisting>
+
+            <para>Given our sum being within range [-3 * -128, 3 * 127]
+            dividing by three guarantees <code language="java">avg</code> to
+            be within [-128, 127]. The <code language="java">(byte)</code>
+            cast will thus safe with respect to overflow problems of our
+            returned value.</para>
           </answer>
         </qandaentry>
       </qandadiv>