The Harmonic Mean is what you use to average rates. It is calculated by:
•1: Inverting all your rates
•2: Adding the inversions together
•3: Inverting the resulting sum
•4: Multiplying by 'n' (in this case 8).
That is the Harmonic Mean, or the average rate. If you don't believe me, Google "Harmonic Mean" for more details.
When you do all that, you get 145.084 as your average rate.