solution added

Author: martin-henz

xml/chapter3/section3/subsection4.xml

@@ -720,8 +720,32 @@ function or_gate(a1, a2, output) {
       <JAVASCRIPT><JAVASCRIPTINLINE>inverter_delay</JAVASCRIPTINLINE>?
       </JAVASCRIPT>
     </SPLITINLINE>
-  </EXERCISE>
+    <SOLUTION>
+      (contributed by github user <LINK address="https://github.com/taimoon">taimoon</LINK>)
+      <BREAK/>
+      Idea: ~(~a &amp; ~b) = nand(~a, ~b) = ~~a | ~~b = a | b
+      <SNIPPET EVAL="no">
+	<JAVASCRIPT>
+function nand_gate(a1, a2, out){
+    const tmp = make_wire();
+    and_gate(a1, a2, tmp);
+    inverter(tmp, out);
+}
 
+function or_gate(a1, a2, out){
+    const not_a1 = make_wire();
+    const not_a2 = make_wire();
+    inverter(a1, not_a1);
+    inverter(a2, not_a2);
+    nand_gate(not_a1, not_a2, out);
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      The delay time of the nand-gate is
+      <JAVASCRIPTINLINE>nand_gate_delay = and_gate_delay + inverter_delay</JAVASCRIPTINLINE> and the delay time of the or-gate above is
+      <JAVASCRIPTINLINE>or_gate_delay = nand_gate_delay + inverter_delay = and_gate_delay + 2 * inverter_delay</JAVASCRIPTINLINE>.
+    </SOLUTION>
+  </EXERCISE>
   <SHORT_PAGE lines="1"/>
   <EXERCISE>
     Figure<SPACE/><REF NAME="fig:ripple-carry"/> shows a