355 ÷ 113 is a good approximation for which mathematical constant?

The number π – also known as Archimedes's constant – is defined in geometry as the ratio of a circle's circumference to its diameter (there are many other equivalent definitions). It crops up in formulas in many areas of mathematics and physics. One feature of this intriguing ratio is that it cannot be expressed as a common fraction; its representation as a decimal never ends and never settles into a permanently repeating pattern. Writing the number π out to ten decimal places it is 3.1415926535…

Although there is no fraction that exactly represents π, there are fractions that are commonly used as simple approximations. For example, many people know 22/7. But 22/7 comes to 3.1428571428… which is a rather poor approximation to the true value of π: it is only accurate to three decimal places.

If, instead, 355/113 is used we get 3.1415929203… which is much closer to the true value of π: it is accurate to six decimal places.