Make this without a calculator

head. 1,000 plus 40 is 1,040, plus 1,000 is 2,040, plus 30 is 3,070, plus 1,000 is 3,070, plus 20 is 3,090, plus 1,000 is 4,090. But it’s at this penultimate sum that things get tricky. Most people, when adding 10 to that figure, come up with 5,000. But that’s your brain skipping ahead (and overlooking a zero). The correct sum is, of course, 4,100.continue reading …