I think you're right -- the linkage between the preface sentences and examples can be improved :-).
The "logic" is that sharing knowledge is a foundational value of education and in many respects reuse is dependant on a willingness to share. So we need to tighten up the examples --- I'll take a look and reword a little. Let me know if the result is better and improve where you can.
Total agreement on the revision re order of operations. You can see I don't teach Math ;-)
The revised lead up is much improved. I like that it clearly states that building on existing knowledge is an instance of reuse. I didn't make that connection at first.
I messed a bit more with the math example. It's hard to make the second sentence correct and user-friendly. Maybe we should delete it: people who know algebra will know why its a pre-requisite without our saying why and for people who don't the further explanation seems confusing. Just a thought.
I think the lead-up works better now as well.
I'm OK with dropping the second sentence with the order of operations -- it was included for those who may not be familiar with the concept. Given that its an example -- it should be OK to delete this.
I made another attempt at the example to simplify the explanation. I think this one works OK. I agree it's nice to have a little bit of explanation/example for those unfamiliar with the concept.