Ruby provides a variety of built-in methods you may use on numbers. The following is an incomplete list of <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>integer</a> and <ahref='https://ruby-doc.org/core-2.2.0/Float.html#method-i-ceil'target='_blank'rel='nofollow'>float</a> methods.
Use `.even?` to check whether or not an <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a> is even. Returns a `true` or `false`**boolean**.
Use `.odd?` to check whether or not an <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a> is odd. Returns a `true` or `false`**boolean**.
The `.ceil` method rounds <ahref='https://ruby-doc.org/core-2.2.0/Float.html#method-i-ceil'target='_blank'rel='nofollow'>**floats**</a>**up** to the nearest number. Returns an <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a>.
The `.floor` method rounds <ahref='https://ruby-doc.org/core-2.2.0/Float.html#method-i-ceil'target='_blank'rel='nofollow'>**floats**</a>**down** to the nearest number. Returns an <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a>.
Using `.to_s` on a number (<ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a>, <ahref='https://ruby-doc.org/core-2.2.0/Float.html#method-i-ceil'target='_blank'rel='nofollow'>**floats**</a>, etc.) returns a <ahref='https://ruby-doc.org/core-2.2.0/String.html'target='_blank'rel='nofollow'>string</a> of that number.
## <a href='https://ruby-doc.org/core-2.2.0/Integer.html#method-i-gcd' target='_blank' rel='nofollow'>Greatest Common Denominator</a>:
The `.gcd` method provides the greatest common divisor (always positive) of two numbers. Returns an <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a>.
Use `.round` to return a rounded <ahref='https://ruby-doc.org/core-2.2.0/Integer.html'target='_blank'rel='nofollow'>**integer**</a> or <ahref='https://ruby-doc.org/core-2.2.0/Float.html'target='_blank'rel='nofollow'>**float**</a>.