Motion, matter and energy interact conceptually at every scale in the universe.  
As many of the previous exercises have shown, seeking the answers to complex 
scientific questions require us to combine mathematical equations in many
areas of astronomy.  

Many of the exercises in this part of the book will be linked to other exercises 
as coherent "problem sets."  These sets demonstrate the step-by-step breakdown of 
complex problems, using many different concepts, to reach solutions that provide 
insight into the universe and how it works.  At all times, though, the most 
important tool is neither the equations more the calculator, but the reasoning 
needed to see what questions need to be asked, and how they can be answered.
So in the end, numerical common sense always asserts itself as a crucial part of 
scientific reasoning and discovery.