What would have been our number system if humans had more than 10 fingers? Try to solve this puzzle. Try to solve this puzzle:

The first expedition to Mars found only the ruins of a civilization.
  From the artifacts and pictures, the explorers deduced that the
  creatures who produced this civilization were four-legged beings with
  a tentatcle that branched out at the end with a number of grasping
  "fingers". After much study, the explorers were able to translate
  Martian mathematics. They found the following equation:
  $$5x^2 - 50x + 125 = 0$$
  with the indicated solutions $x=5$ and $x=8$. The value $x=5$ seemed
  legitimate enough, but $x=8$ required some explanation. Then the explorers 
  reflected on the way in which Earth's number system developed, and found
  evidence that the Martian system had a similar history. How many fingers would 
  you say the Martians had?
$(a)\;10$
$(b)\;13$
$(c)\;40$
$(d)\;25$

P.S. This is not a home work. It's a question asked in an interview.

The correct answer is (a) 10.
There is no comment which number system the given answers refer to. As all other numbers refer to the Martian number system, we can safely assume the answers refer to the Martian number system as well.
