Arduino based ultrasonic sensor program, output HIGH to control electric motor
As per C++ naming convention, only user-defined types and macros in C++ are capitalized.  Your constants, global variable, and functions should start with a lowercase letter.
Since your constants are similar, you can group them into an enum instead:

enum Pin { Motor=7, Trigger, Echo };


With the first one set to 7, the following ones will be set to 8 and 9 respectively.

This could also help with code maintenance.  If you ever need to add another Pin, you just need to insert it into the enum appropriately, as opposed to creating another constant.
Duration is only used in loop(), so just initialize and use it there.  Having it as a global variable could invite bugs, and is otherwise bad practice.  This applies to global variables in general because they're accessible throughout the entire program, meaning they can be changed anywhere.
You're using "magic numbers" (hard-coded numbers) in places.  You should either make them variables or constants, or provide comments to specify their meaning.
Your indentation is quite inconsistent.  You indent a different number of spaces in different functions, but you should stick to one (most common is four spaces).  You especially should have indentation within functions, otherwise it may appear that the code isn't contained within that function.
This isn't syntactically correct:

if  (dist_avg &lt; 600 &amp;&amp; dist_avg &gt; 400);
digitalWrite(MotorPin,HIGH);
else
digitalWrite(MotorPin,LOW);


There shouldn't be a semicolon after the if condition, otherwise the following statements won't work with it.  The second and fourth lines should also be indented.