<PRE><B>Question 152556</B>
One equation might be particularly useful:
Speed * Time = Distance
&lt;br&gt;We can shorten these into variables we need:
speed of the express train = s&lt;sub&gt;e&lt;/sub&gt;
speed of local train = s&lt;sub&gt;l&lt;/sub&gt;
&lt;br&gt;Using the information from the problem. Both trains traveled the same time. We also know the total distance:
time = 4 hours
total distance = 480 miles
&lt;br&gt;Now let&#39;s use the information we know. Since the express train travels twice as fast as the local train, we can set the speed of the express train in terms of the local train:
s&lt;sub&gt;e&lt;/sub&gt; = 2&lt;sub&gt;l&lt;/sub&gt;
&lt;br&gt;Let&#39;s plug it all back into the original equation:
Speed * Time = Distance
(s&lt;sub&gt;e&lt;/sub&gt; + s&lt;sub&gt;l&lt;/sub&gt;) * 4 = 480
(s&lt;sub&gt;e&lt;/sub&gt; + s&lt;sub&gt;l&lt;/sub&gt;) = 120
&lt;br&gt; Since s&lt;sub&gt;e&lt;/sub&gt; = 2&lt;sub&gt;l&lt;/sub&gt;
2s&lt;sub&gt;l&lt;/sub&gt; + s&lt;sub&gt;l&lt;/sub&gt; = 120
3s&lt;sub&gt;l&lt;/sub&gt; = 120
s&lt;sub&gt;l&lt;/sub&gt; = 40 mph
&lt;br&gt;The average speed of the local train is 40 mph. 
&lt;br&gt;Now we can solve for the speed of the express train.
s&lt;sub&gt;e&lt;/sub&gt; = 2&lt;sub&gt;l&lt;/sub&gt;
s&lt;sub&gt;e&lt;/sub&gt; = 2(40)
s&lt;sub&gt;e&lt;/sub&gt; = 80 mph
&lt;br&gt;The average speed of the express train is 80 mph.


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