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ADDITION AND SUBTRACTION
Addition. — The greater part of all practical calculation is addition. It is of the utmost importance that you add with ease, precision, and speed, but skill in this comes only by much practice. This book aims to give you both practice and principle — problems to solve and methods of solving them.
Addition is the process of finding a number which is equal to two or more numbers taken together. The result obtained by adding is called the sum.
Rule. — Group or combine two or more numbers (mentally) and name results only.
In the example given, instead of saying EXAMPLE
and two are nine, and seven are sixteen,” and so on, 6 4 7 omit as far as possible all unnecessary words and 5 9 2 repetitions. 7 9 7 Say simply, or rather think (first column), "nine, 8 6 8 sixteen, twenty-four, thirty-two, thirty-six ; (second 6 4 8 column), seven, sixteen, twenty-five, thirty-one, thirty
8 3 4 five, thirty-eight; (third column), nine, fourteen, 4 3 8 6 twenty-one, twenty-nine, thirty-five, forty-three."
Be constantly on the alert for simple combinations of numbers and thus lessen the labor of addition. For instance, if you have a 6 and a 5 in a column, you know that they make 11, and if your sum up to that point is 27, it is much easier to think 38 than it is to take the two intermediate steps, each of which is more difficult than the complete step.
Always add the number to be carried to the next column on commencing, and if the columns are long, write it down, as it may, save the trouble of going over work already performed. Take pride and pains in making neat, plain, and legible figures.
Check. — Add in the opposite direction, upward or downward.
This is preferable to doing the work over a second time in the same way, as a mistake once made is likely to be repeated.
1 Always check your work, no matter how expert you are in computation.