A 8 ft tall cardboard box standing next to a man casts a 20 ft shadow. If the man casts a shadow that is 15 ft long, then how tall is he?

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