Full Idea
The 'distributive' law says you will get the same result if you first add two numbers, and then multiply them by a third, or first multiply each by the third and then add the results (i.e. a · (b+c) = a · b + a · c ).
Gist of Idea
'Distributive' laws say if you add then multiply, or multiply then add, you get the same result
Source
R Kaplan / E Kaplan (The Art of the Infinite [2003], 2 'Tablets')
Book Reference
Kaplan,R and Kaplan,E: 'The Art of the Infinite' [Penguin 2004], p.35
A Reaction
Obviously this will depend on getting the brackets right, to ensure you are indeed doing the same operations both ways.