Elementary arithmetic, with brief notices of its history... by Robert Potts.

2
dividing the number by 12, and shillings to pounds by dividing thle
number by 20.1
3. PROB.-To find the sum and the difference of suGms of money.
In the processes of addition and subtraction of money expressed in
pounds, shillings, pence, and farthings, the same principle is employed
as in the addition and subtraction of abstract numbers, with this difference. In the latter, 10 units of any order make 1 of the next superior
order; but in the units of English money, 4 of the smallest units
make 1 of the second or next greater; 12 of the second units make 1
of the third; and 20 of the third units make 1 of the fourth or the
greatest unit. Hence in adding the different units of money beginning
with the smallest; for every 4 farthings, 1 must be added to the pence;
for every 12 of the pence, 1 must be added to the shillings; and for
every 20 of the shillings, 1 must be added to the pounds. The same
1 Ex. To find how many farthings there are in ~16 14s. 9 d.
~ s. d.
16 14 94 -20 shillings in one pound.
334 shillings in ~16 14s.
12 pence in one shilling.
4017 pence in ~16 14s. 9d.
4 farthings in one penny.
16069 farthings in ~16 14s. 9cd.
Conversely. How many pounds, shillings, pence, and farthings are there in
16069 farthings?
4)16069 farthings.
12) 4017 pence and 1 farthing over, or.
20) 334 shillings, and 9 pence over.
16 pounds and 14 shillings over.
Hence 16069 farthings are equal to ~16 14s. 9~d.
2 Ex. To find the sum of ~53 12s. 8~-d. and ~25 15s. 9|d.
~ s. d.
53 12 81 -25 15 94
~79 8 61 sum.
The process may be thus described:
First. 3 farthings and 2 farthings are 5 farthings, which are equal to 1 penny
and; reserve the id.
Secondly. 1 penny, 9 pence, and 8 pence are 18 pence, which are equal to 1
shilling and 6 pence; reserve the 6d.
Thirdly. 1 shilling, 15 shillings, and 12 shillings are 28 shillings, which are equal
to 1 pound and 8 shillings; reserve the 8s.
Fourthly. 1 pound, 5 pounds, and 3 pounds are 9 pounds.
Lastly. 20 pounds and 50 pounds are 70 pounds.
Hence, the sum is 79 pounds 8 shillings 6 pence and 1 farthing.
rTh'. process of the addition of money may be facilitated by committing to
memory the numbers of shillings and pence in all the numbers of pence between 12
and 200, as also the number of pounds and shillings in all the numbers of shillings
between 20 and 200.