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.
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.
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, add two ciphers to the multiplicand, and add to this the multiplicand.
To multiply by 1001, add three ciphers to the multiplicand, and add to this the multiplicand.
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.
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.