What decimal is less than 0.8 is it 0.9, 0.79, 0.81, or 0.85

Problem number 26 of the rhind papyrus says: find a quantity such that when it is added to of itself the result is a 15. the modern day equation that models this problem is x + x = 15. what is the solution to the equation? x = 10 x = 12 x = 15 x = 30