How To Become A Lightning Calculator
Accuracy should be first considered, then rapidity. Quick adders, by the way, are the most accurate.
Write the numbers in vertical lines, avoiding irregularity. This is important. Keep your thought on results not numbers themselves. Do not reckon 7 and 4 are 11 and 8 are 19, but say 7, 11, 19 and so on.When the same number is repeated several times, multiply instead of adding.
When adding horizontally begin at the left.
In adding long columns, prove the work, by adding each column separately in the opposite direction, before adding the next column. Many accountants put down both figures as in the illustration. The sum of the first column is 12; carrying one, the sum of the second is 20; carrying two, the sum of the third column is 15; carrying one, the sum of the fourth column is 21, and the total, 21502, is found by calling off the last two figures and the right-hand figures, following the wave line in the illustration. This method is better than the old one of penciling down the number to carry. If one desires to go back and add a certain column a second time, the number to carry is at hand and the former total is known.
How to Add Two Columns at Once.
To the inexperienced it will be a difficult task to add two columns at once, but many of those who have daily practice in addition find it about as easy to add two columns as one. Say 99 and 50 are 149, and 6 are 155, and 10 and 50 are 215 and 3 are 218, and 12 are 230. Carry 2, and say 33 and 12 are 45, and 20 are 65, and 6 are 71, and 30 are 101, and 2 are 103, and 23 are 126.
Much of the information here contained is compiled from W. D. Rowland’s valuable little volume, entitled “How to become expert with figures.” You can get this handy book by sending 25 cents in stamps to American Nation Co., Boston.
To Multiply Any Number by 11.
Write the first right-hand figure, add the first and second, the second and third, and so on; then write the left-hand figure. Carry when necessary.
219434 × 11 = 2413774
Put down the right-hand figure 4. Then say, 4 and 3 are 7; then, 3 and 4 are 7; then, 4 and 9 are 13, put down 3 and carry 1; then, 9 and 1 and 1 are 11, put down the 1 and carry 1; then, 1 and 2 and 1 are 4; then write the left-hand figure 2. In multiplying small numbers, such as 24 by 11, write the sum of the two figures between the two figures, making 264, the required product.
To Multiply by 101, 1001, etc.
To multiply by 101, add two ciphers to the multiplicand, and add to this the multiplicand.
2341 × 101 = 234100 + 2341
To multiply by 1001, add three ciphers to the multiplicand, and add to this the multiplicand.
To Multiply by 5, 25, 125.
To multiply by 5, add a cipher and divide by 2.
To multiply by 25, add two ciphers and divide by 4.
To multiply by 125, add three ciphers and divide by 8.
Another Easy Way to Multiply.
To multiply two figures by two figures, proceed as follows: Multiply units by units for the first figure.
Carry and multiply tens by units and units by tens, (adding) for the second figure. Carry and multiply tens by tens for the remaining figure or figures. In this example proceed as follows:
2 × 4 = 8 = 1st figure.
(4 × 8) + (5 × 2) = 42. Therefore 2 = 2d figure.
(5 × 8) + 4 carried = 44 = 3d and 4th figures.
By a little practice any one may become as familiar with this rule and as ready in its application as with the ordinary method.
To multiply any number by 2 1/2, add one cipher, and divide by 4.
To multiply any number by 3 1/3, add one cipher, and divide by 3.
To multiply by 33 1/3, add two ciphers, and divide by 3.
To multiply any number by 1 3/7, add one cipher, and divide by 7.
To multiply by 16 2/3, add two ciphers, and divide by 6.
To multiply by 14 2/7, add two ciphers, and divide by 7.
To multiply by 875, add three ciphers, and divide by 8.
To divide by 25, multiply by 4, and cut off two figures.
To divide by 125, multiply by 8, and cut off three figures.
To multiply by 12 1/2, add two ciphers, and divide by 8.
To find the value of any number of articles at 75 cents each, deduct one-quarter of the number from itself and call the remainder dollars.
To Subtract Any Number Consisting of Two Figures from 100.
Take the first figure from 9, and the second from 10. For example: in subtracting 73 from 100, or in taking 73 cents change out of a dollar, say 7 from 9 and 2, and 3 from 10 and 7, or 27 cents. Practice this rule. It is simple, and will be found particularly helpful in making change.
A number is divisible by 2 when the last digit is even.
A number is divisible by 4 when the last two digits are divisible by 4.
To divide by 12 1/2, multiply by 8, and cut off two figures.
Simple Discount Rule.
This simple rule is in use in many houses where several discounts are allowed from list prices. Suppose the list price of a piano to be $500, and you allow an agent 25, 20 and 10 off.
Subtract each from 100 and multiply and you get .54. $500 × .54 = $270, the agent’s price.
Multiply the principal (amount of money at interest) by the time, reduced to days; then divide this product by the quotient obtained by dividing 360 (the number of days in the interest year) by the per cent. of interest, and the quotient thus obtained will be the required interest. Require the interest of $462.50 for one month and eighteen days at 6 per cent. An interest month is 30 days; one month and 18 days equals 48 days. $462.50 multiplied by .48 gives $222.0000; 360 divided by 6 (the per cent. of interest) gives 60, and $222.0000 divided by 60 will give you the exact interest, which is $3.70. If the rate of interest in the above example were 12 per cent., we would divide the $222.0000 by 30 (because 360 divided by 12 gives 30); if 4 per cent., we would divide by 90; if 8 per cent., by 45; and in like manner for any other per cent.