Target/Content=Result
A better question to ask might be, what problem does responsive web design solve? Well, as you may have noticed, computers aren't the only piece of hardware with a web browser anymore. I might get myself in trouble by saying this, but the iPhone was one of the first mobile devices to feature a really great web browser, and it really put the spotlight on upgrading the experience of the mobile web. Many other devices followed suit and, seemingly overnight, the face of the mobile web had changed.
Floating Point
If your values don't work out so neatly, and you get some floating point value with many numbers after the decimal, don't round the value! We humans may enjoy nice neat numbers and making our code look pretty, but your computer (and the final look of your design) will benefit from the seemingly excessive mathematical precision. Fluid grids are a very important part of creating a responsive design, but they can only take us so far. When the width of the browser becomes too narrow, the design can start to severely break down. For example, a complex three-column layout isn't going to work very well on a small mobile phone. Fortunately, responsive design has taken care of this problem by using media queries.