A helicopter was 120 feet above sea level. After descending into a canyon, it was 150 feet below sea level. What was the change in the helicopter’s elevation?

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