Thanks for pointing this out but - honestly - this whole challenge is utterly pointless. Writing a programme to calculate factorials without any additional requirement (like arbitrary large number or such) is not proving anything. If one knows that the input is between 1 and 15 the best solution is probably a lookup table. And there is an explicit formula that states the solution to two simultaneous equations, so programming a full blown algorithm for solving them is a waste of time as well.
It looks really like a lure to get people to apply for trial accounts...