Following each step in order, 1000 plus 40 gives 1040. Adding 1000 reaches 2040, then 30 brings it to 2070. Another 1000 produces 3070, plus 20 makes 3090. The third 1000 pushes the total to 4090, and the final addition of 10 — not 1000 — lands the sum at 4100.
The arithmetic itself is straightforward: as the original explanation puts it, “Four groups of 1000 make 4000. The smaller amounts — 40, 30, 20, and 10 — make 100.” Combined, those two totals equal 4100.
Why the brain defaults to 5000 instead of the right answer
The wrong answer is not random. After processing three additions of 1000 in a row, the brain builds an expectation: the next significant number will also be 1000. When the final value is actually 10, that expectation overrides what is written, and the mind substitutes the pattern for the fact.
This is the brain’s tendency to seek round numbers and familiar sequences. The puzzle exploits that shortcut directly — the structure is designed so that pattern-matching produces a plausible but incorrect result.
The mental-only instruction amplifies the effect. Working without pen and paper removes the natural pause that written calculation provides, making it easier to rush through the sequence and lock in the wrong total before double-checking.
Why mental math is different from written calculation
When solving a problem on paper, the act of writing each number down creates a natural pause that slows the process and reduces errors. Mental arithmetic removes that pause entirely, leaving the brain to rely on shortcuts and pattern recognition — mechanisms that are efficient in everyday life but can misfire on sequences specifically designed to exploit them.
