pence in both lines to it. Reduce the sum to shillings and pence; set down the last only, and carry the first to the line of shillings, and so on. The same method must be followed when the quantities are of any other sort; and if the tables be kept in memory, the process will be easy.
- Subtraction is performed on the same principle as in (40), namely, that the difference of two quantities is not altered by adding the same quantity to both. Suppose it required to subtract £19 . 13. 10¾ from £24. 5. 7½. Write these quantities under one another thus:
- £24. 5. 7½
-
- 10¾
-
Since ¾ cannot be taken from ½ or ²/₄, add 1d. to both quantities, which will not alter their difference; or, which is the same thing, add 4 farthings to the first, and 1d. to the second. The pence and farthings in the two lines then stand thus: 7⁶/₄d. and 11¾d. Now subtract ¾ from ⁶/₄, and the difference is ¾ which must be written under the farthings. Again, since 11d. cannot be subtracted from 7d., add 1s. to both quantities by adding 12d. to the first, and 1s. to the second. The pence in the first line are then 19, and in the second 11, and the difference is 8, which write under the pence. Since the shillings in the lower line were increased by 1, there are now 14s. in the lower, and 5s. in the upper one. Add 20s. to the upper and £1 to the lower line, and the subtraction of the shillings in the second from those in the first leaves 11s. Again, there are now £20 in the lower, and £24 in the upper line, the difference of which is £4; therefore the whole difference of the two sums is £4. 11. 8¾. If we write down the two sums with all the additions which have been made, the process will stand thus:
- £24 . 25 . 19⁶/₄
- 20 . 14 . 11¾
- Difference £4 . 11 . 8¾
- The same method may be applied to any of the quantities in the tables. The following is another example:
From 7 cwt. 2 qrs. 21 lbs. 14 oz. Subtract 2 cwt. 3 qrs. 27 lbs. 12 oz.
After alterations have been made similar to