Thanks Alan,
I can see that a denominator of 5 works for your example but I still don't comprehend where
$$\int1 dx$$ comes from. Why is it 1? What are you finding the sum of?
I obvioulsy have a major gap in my understanding.
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Actually I am just thinking about the units as rates The average of anything can be thought of as whatever per unit or item. That makes sense. So thinking of it like this I absolutely get the difference on the bottom.
For example (for the benefit of others) The average of 6m,3m and 2m = 9m/3items = 3m/item
So $$\int1 dx$$ is the sum of units on the x axis which is 5.
So for the given question the average is sum of the gradients/unit of x
Does that make sense, sort of anyway?
Maybe I have got it! 