Add the numbers in the order they are given. Start with 1000. Add 40 and you have 1040. Add 1000 and you have 2040. Add 30 and you have 2070. Add 1000 and you have 3070. Add 20 and you have 3090. Add 1000 and you have 4090. Add 10 and you have 4100. Four groups of 1000 make 4000. The smaller amounts—40, 30, 20, and 10—make 100. Four thousand plus one hundred is 4100. Nothing in the problem adds another thousand at the end.

The 5000 mistake happens because the brain likes round numbers and familiar patterns. After several jumps of 1000, the final 10 gets inflated into another 1000, or 4100 gets rounded up because 5000 is the number that “sounds finished.” Viral posts reward the first confident comment, not the careful one. The phrase “just your mind” makes people skip the only method that works: one step at a time.

The lesson is the same as any rushed problem. The puzzle is not testing genius. It is testing whether you will stay with the actual list instead of the story you invent halfway through. The result is 4100. Write it down in your head after every line and the trick disappears. The answer was never hidden. It was only waiting for someone to add what was written, and nothing more.