Anna browne bought a suit at a 25% off sale. the suit had been marked $100. how much did she save by buying on sale ?

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