I created the convenience functions "multiple_of_3" and "multiple_of_5" for a couple of reasons:

1. To make it clear to the interviewer that I "get" the problem. 2. To prevent myself from making any dumb mistakes. I'm only a half-decent coder, so I've made it a habit of putting such "Mollyguards" (http://en.wiktionary.org/wiki/molly-guard) in my code as a precaution. 3. To show a little Ruby coding style. The convention in Ruby is to favour lots of concise little routines over big-ass ones. See "Eloquent Ruby" for the guideline and Shopify's ActiveMerchant library for an example of such Ruby in actual production code.