The Lightning Calculator’s Addition.
Reading edition
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There are experts who can add very rapidly. The best of them, however, cannot add up a column of ones any faster than you can. Here is how some of the rapid addition is performed. The operator writes a line of figures, then another, and so on. The second line, however, added to the first makes nines, except at the extreme right, where the two figures add to ten. The third and fourth bear the same relation, and as many more as he chooses to put down. The last two lines, however, are put down at random. Now, to add these columns, he begins anywhere, perhaps at the left-hand side, putting down 2 (the number of pairs above), then by simply adding the two bottom lines, he gets the correct sum.