These
modes are several.
(a.) "Doubling a number, and then adding or subtracting, as the case may
require. For instance, in the example already specified, in order to add
seven and eight, you say, 'Twice seven are fourteen, and one are
fifteen'" ("Yes, sir, yes, sir"); "or, 'Twice eight are sixteen, and
taking one off leaves fifteen." ("Yes, sir.")
(b.) Another way of calculating is to skip about the column, adding
those numbers which you can combine most easily, and then bringing in
the rest as you best can. Thus, if you see three eights in one column,
you say, 'Three times eight are twenty-four,' and then you try to bring
in the other numbers. Often, in such cases, you forget what you have
added and what you have not, and get confused ("Yes, sir"), or you omit
something in your work, and consequently it is incorrect.
(c.) If nines occur, you sometimes add ten, and then take off one, for
it is very easy to add ten.
(d.) Another method of calculating, which is, however, not very common,
is this: to take our old case, adding eight to seven, you take as much
from the eight to add to the seven as will be sufficient to make ten,
and then it will be easy to add the rest. Thus you think in a minute
that three from the eight will make the seven a ten, and then there will
be five more to add, which will make fifteen. If the next number was
seven, you would say five of it will make twenty, and then there will be
two left, which will make twenty-two.
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