Averages & Population Density

My ecology textbook describes three types of averages used to describe an aspect of population density. I am caught up on how to calculate one of these types. The example situation give in the text is as follows:

*There is a population of 1000 insects that is exploiting 100 plants as resources. There are 91 insects on each of 10 plants, and 1 insect each on the remaining 90 plants.*

- The first type of average is just the mean number of insects per plant, which is 10. I've got that.

- The second type of average is organism-weighted, or the mean number of insects that an individual has to share its plant with. Which is to say: [(
**91** individuals***10** plants*a density of **91** experienced by each of those individuals) + (**1** individual***90** plants*a density of **1** experienced by each of those individuals)]/**1000** insects. This equals an average density of 82.9 individuals. I understand this too.

- But then, the text describes a third type of average, and this is the one that I do not understand how to calculate. It describes this average as the average density of insects experienced by the plants, a.k.a. "exploitation pressure", and says that this average is 1.1 insects per plant. The book does not describe how they arrived at this number, and I haven't been able to figure it out. What am I missing?