By far the most popular and well-stocked category of used instruments is our selection of guitars, where we've got a huge variety of all the big names. Used Gibson Les Paul, pre-owned Fender Telecaster and secondhand Stratocaster guitars are commonly found within these pages, waiting for their new homes with players and collectors.
22/7 is a pretty decent choice for teaching 11-year-olds about fractions that are close to pi. 355/113 would be better, if you think they can remember numbers that big. And approxRational is fun. Exercise for the interested reader: implement an inverse to approxRational.
The number [math]\pi[/math] is not equal to [math]\frac{22}{7}[/math]. Thus, [math]\pi\ne\frac{22}{7}[/math]... in fact, [math]\frac{22}{7}[/math] is an overestimate of [math]\pi[/math]. Nevertheless, it is a reasonable approximation of [math]\pi[/math] if you don't need a lot of accuracy.