Plzz quiz (online) if one angle of a set of alternate interior angles on parallel lines measure 77 then the other angle also equals 77 always sometimes never

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