Yes, don't think about that stuff. Let's say we have x and y. Why sum(x) / sum(y) or avg(x) / avg(y) is different from avg(x / y)?

The last method now seems goofy but why is it right or wrong? And is the difference just a rounding error or avg (x / y) calculates something different?

But method 3 gives equal weight to the tasks no matter how quickly or slowly they run. So you get 18.333 c/h.

but depending on what you want to achieve, if you want a more robust statistic you could possibly use the medians instead, or even truncated means to chuck out outliers.

As for the problem with the car above, the answer isn't as simple as I thought http://en.wikipedia.org/wiki/Harmonic_mean#In_physicsWell, I guess we need a professional statistician