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The rule of PRACTICE is so called from the great facility it affords in calculation, and from its general use among merchants and tradesmen. It is a short and concise method of computing the value of commodities by taking aliquot parts. An aliquot part of a number is such a part as, being taken a certain number of times, will produce the number exactly; thus 6s. 8d. is the third, or aliquot part of a pound; 3d. is the fourth, or aliquot part of a shilling, and so on, as demonstrated in the foregoing Tables.

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5

cent. annum is at the rate of 1d.

2 per cent. per annum is at the rate of d. ditto; hence to find the interest or discount at 5

month for £1,

Goods at

cent. is very easy.

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The sum total of 100,000 farthings, in money of account, is ascertained

by dividing by 4, 12, and 20: thus

4)100,000 farthings.

12)25,000 pence.

20) 2,083 shillings.

£104 3s. 4d.

The Root of any power is such a number as, being multiplied into itself a certain number of times, will produce that power.

Roots... 1 2 3 4 5 6

7 8 9 10 11 12

Squares 14 9 16 25 36

49

64 81 100 121 144

Cubes... 1 8 27 64 125 216 343 512 729 1000 1331 1728

RULES FOR MENTAL CALCULATION.

MULTIPLICATION.

(1) To Multiply by 10.

Add a nought.

6 x 10 = 60.

(2) To Multiply by any number of tens-20, 30, etc. Add a nought, and multiply by the number of tens. 6 × 20 = 60 × 2 =

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120.

(4) To Multiply by any number of hundreds-200, 300, etc. Add two noughts, and multiply by the number of hundreds. 6 × 200 600 × 2 =

=

1200.

(5) To Multiply by any number of tens, hundreds, or thousands.

Add the nought or noughts, and multiply by the number of tens, hundreds, or thousands.

[blocks in formation]

(7) To Multiply by 15, 35, 45, or 55.

Multiply by twice the multiplier, and divide by 2.

[blocks in formation]

(8) To Multiply by 75 (75

300 ÷ 4).

6 x 75

450.

=

Add 2 noughts, multiply by 3, and divide by 4.

[blocks in formation]

=

=

(9) To Multiply by 125 (125 1000 ÷ 8). Add 3 noughts, and divide by 8.

[blocks in formation]

=

(10) To Multiply by 250 (250 = 1000 ÷ 4). Add 3 noughts, and divide by 4.

6 X 250 6000 4 1500.

=

(11) To Square a number (to multiply it by itself).

Add the number to its unit figure, multiply by the tens, and add the square of the unit figure.

Square 23

= 23 X 23.

[blocks in formation]

Cut off the end figure, which will be the remainder.

[blocks in formation]

Cut off the two end figures, which will be the remainder.

[blocks in formation]

(3) To Divide by 1000.

Cut off the three end figures, which will be the remainder.

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(6) To Divide by any number of tens, hundreds, or thousands.

Cut off as many end figures as there are noughts, and divide by the number of tens, hundreds, or thousands.

[blocks in formation]

CALCULATION OF PRICES.

(1) To find the price of a dozen articles.

Call the pence shillings, and every farthing 3d. 1 dozen articles at 3d. = 38.

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When the price exceeds a shilling, reduce shillings to pence. 1 doz. articles at 1s. 61d. = 1 doz. at 181d.

(2) To find the price of any number of dozens.

= 188. 3d.

Find the price of one dozen, and multiply by the number of dozens.

5 dozen articles at 2s. 1d. =25s. 6d. 5 A gross is 144 or 12 dozen.

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=

£6 7s. 6d.

3s. 6d. X 12 = £2 2s.

(3) To find the price of one article, the price of a dozen being given.

Call the shillings pence, and every threepence a farthing. The price of one article at 10s. 3d. per dozen

(4) To find the price of 20 articles.

=

101d.

Call the shillings pounds, and every threepence five shillings.

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(5) To find the price of 240 articles.

Call the pence pounds, and every farthing five shillings.

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(6) To find the price of 480 articles.

Call the pence pounds, and multiply by 2.

480 articles at 5d.

= £5 10s. X 2

=

£11.

(7) To find the price of 960 articles.

Reduce the price of one to farthings, and call farthings

pounds.

960 articles at 24d.

=

10 farthings

= £10.

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