Add numbers chapter

Author: paulgoetze

2_Numbers/1_Types_of_numbers/solution_spec.rb

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+# Types of numbers
+
+---
+
+*You will learn:*
+- which types of numbers are available in Ruby
+- how to create a certain type of number
+- different ways to create a number
+
+---
+
+Ruby has following types of numbers: *Integer*, *Float*, *Rational*, and *Complex*.
+
+## Integers
+
+An *Integer* represents a *whole number*.
+You can create an Integer by declaring the number, like `2000` or `-42`.
+Ruby allows you to write more readable numbers by using underscores, e.g. `1_000_000`.
+Using the underscore style does not change the value of an integer, so `2_000` is
+perfectly the same as `2000`.
+
+Another way to create an Integer is by calling `Integer()` with a parameter, e.g.:
+
+`  Integer(3)`   # => 3
+`  Integer('3')` # => 3
+
+Internally, Ruby uses two different classes for Integers. Dependent on how big the
+Integer is it uses a *Fixnum* or a *Bignum*. You don't have to worry about which class to
+use, Ruby is taking care of this for you!
+
+However, you can check which class Ruby uses for your number by calling the `class`
+method on your number. Open a new terminal tab and open the interactive Ruby console
+with `irb`. Then check the class of different numbers:
+
+`  1000.class`                      # => *Fixnum*
+`  4_611_686_018_427_387_904.class` # => *Bignum*
+
+If you want to learn more, read about Fixnum and Bignum here:
+- http://patshaughnessy.net/2014/1/9/how-big-is-a-bignum
+- (ruby-doc core: Integer)
+
+## Floats
+
+A *Float* represents an *inexact real number*.
+You can create a Float by declaring a number with a decimal place, like `3.0`
+or `-2_000.25`.
+Another way to create a Float is to call `Float()` with a parameter, e.g.:
+
+`  Float(3)`   # => 3.0
+`  Float('3')` # => 3.0
+
+Read more about Float here: (ruby-doc core: Float).
+
+## Rationals
+
+A *Rational* represents a rational number (a paired integer number a/b, where b > 0).
+You can create a Rational by calling `Rational()` with one or two parameters, e.g.:
+
+`  Rational(2)`     # => *(2/1)*
+`  Rational(2, 3)`  # => *(2/3)*
+`  Rarional('2/3')` # => *(2/3)*
+
+Read more about Rational here: (ruby-doc core: Rational).
+
+## Complex numbers
+
+A *Complex* represents a complex number.
+You can create a complex number by calling `Complex()` with one or two parameters, e.g.:
+
+`  Complex(2)`          # => *(2+0i)*
+`  Complex(2.1, 0.2)`   # => *(2.1+0.2i)*
+`  Complex('2.1+0.2i')` # => *(2.1+0.2i)*
+`  Complex('2.1@0.2')`  # => *(2.0581398134666076+0.4172055946696286i)* - polar form
+
+Read more about Complex here: (ruby-doc core: Complex).
+
+---
+
+Create some numbers of the different described types in the interactive Ruby console
+(`irb`) and press *v* to mark this unit green if you are done!
+
+---

2_Numbers/1_Types_of_numbers/task.md

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+# Basic operators
+
+---
+
+*You will learn:*
+- how to use basic arithmetic operators on numbers
+
+---
+
+You can apply

2_Numbers/2_Basic_operators/solution_spec.rb

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+# Converting numbers
+
+---
+
+*You will learn:*
+- how to convert numbers to other number formats
+
+---