Question 1139149
<pre>
              SPEED            TIME           DISTANCE

Luke          {{{21/7=3}}}                           {{{4&1/2}}}

Dad           {{{21/4=5&1/4}}}                         {{{4&1/2}}}
</pre>

Time quantities easily filled in, {{{T=D/R}}}.

<pre>
              SPEED            TIME           DISTANCE

Luke          {{{21/7=3}}}         {{{1&1/2}}}                {{{4&1/2}}}

Dad           {{{21/4=5&1/4}}}       {{{6/7}}}               {{{4&1/2}}}
</pre>

The extra time Luke needed compared to his dad,  {{{1&1/2-6/7=9/14}}}{{{hours}}}.  If dad reached top at noon, then Luke reached top  {{{9/14}}} hours later, or 0.64286 hour, or 39 minutes.