solution: Let $x = \cos \left( \frac{2 \pi}{15} \right) \cos \left (\frac {4 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right).$  Then by repeated application of the double angle formula,
  \begin{align*}
  x \sin \left( \frac{2 \pi}{15} \right) &= \sin \left( \frac{2 \pi}{15} \right) \cos \left( \frac{2 \pi}{15} \right) \cos \left (\frac {4 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right) \\
  &= \frac{1}{2} \sin \left( \frac{4 \pi}{15} \right) \cos \left (\frac {4 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right) \\
  &= \frac{1}{4} \sin \left (\frac {8 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right) \\
  &= \frac{1}{8} \sin \left( \frac{16 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right) \\
  &= \frac{1}{16} \sin \left( \frac{32 \pi}{15} \right) \\
  &= \frac{1}{16} \sin \left( \frac{2 \pi}{15} \right),
  \end{align*}so $x = \boxed{\frac{1}{16}}.$
prompt.content: <|im_start|>system
  You are a helpful assistant.<|im_end|>
  <|im_start|>user
  Simplify
  \[\cos \left( \frac{2 \pi}{15} \right) \cos \left (\frac {4 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right).\]
  Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
  <|im_start|>assistant
  
prompt.role: user
extra_info.answer: \frac{1}{16}
extra_info.question: Simplify
  \[\cos \left( \frac{2 \pi}{15} \right) \cos \left (\frac {4 \pi}{15} \right) \cos \left( \frac{8 \pi}{15} \right) \cos \left( \frac {16 \pi}{15} \right).\]
extra_info.split: test