Once you know the actual sum, you can do it either:
sum all, subtract total - sum.
Or with data structures:
it would be a variant of the "given an array find all pairs of numbers that add up to a sum". The benefit of using the data structure approach would be that you can now find x missing numbers in the range 1 - 250.
note* the closed form might have a off by one error
There's a higher level of photographic memory that very few people are capable of. Those that can call on demand images of the past at will. I've met one, and knew of a teacher who knew one. Like the author of this article stated. It can be a curse, nightmares and things they don't want to remember stick with them. Forever.
Although a lot of people also have really great memory, which can arguably said to be "somewhat photographic" as the author said. In which case, I do as well. I cannot agree that being able to walk through a previous scene with a bird's eye view is part of being photographic memory though. I think that as just powerful spatial visualization combined with good memory.
In theory as they said, it's reasonable and it makes sense. However getting it to work in practical is usually a whole other story. Humanity will see it eventually I'm sure.
Creative and fun though