of ways in which 5,940 may be the difference of two squares—­that is, 8.  To find these eight squares, as it is an even number, we first divide by 4 and get 1485, the eight pairs of factors of which are 1 x 1485, 3 x 495, 5 x 297, 9 x 165, 11 x 135, 15 x 99, 27 x 55, and 33 x 45.  The sum and difference of any one of these pairs will give the required numbers.  Thus, the square of 1,486 less the square of 1,484 is 5,940, the square of 498 less the square of 492 is the same, and so on.  In the case of 63 above, the number is odd; so we factorize at once, 1 x 63, 3 x 21, 7 x 9.  Then we find that half the sum and difference will give us the numbers 32 and 31, 12 and 9, and 8 and 1, as shown in the solution to the puzzle.

The reverse problem, to find the factors of a number when you have expressed it as the difference of two squares, is obvious.  For example, the sum and difference of any pair of numbers in the last sentence will give us the factors of 63.  Every prime number (except 1 and 2) may be expressed as the difference of two squares in one way, and in one way only.  If a number can be expressed as the difference of two squares in more than one way, it is composite; and having so expressed it, we may at once obtain the factors, as we have seen.  Fermat showed in a letter to Mersenne or Frenicle, in 1643, how we may discover whether a number may be expressed as the difference of two squares in more than one way, or proved to be a prime.  But the method, when dealing with large numbers, is necessarily tedious, though in practice it may be considerably shortened.  In many cases it is the shortest method known for factorizing large numbers, and I have always held the opinion that Fermat used it in performing a certain feat in factorizing that is historical and wrapped in mystery.

140.—­FIND ADA’S SURNAME.

The girls’ names were Ada Smith, Annie Brown, Emily Jones, Mary
Robinson, and Bessie Evans.

141.—­SATURDAY MARKETING.

As every person’s purchase was of the value of an exact number of shillings, and as the party possessed when they started out forty shilling coins altogether, there was no necessity for any lady to have any smaller change, or any evidence that they actually had such change.  This being so, the only answer possible is that the women were named respectively Anne Jones, Mary Robinson, Jane Smith, and Kate Brown.  It will now be found that there would be exactly eight shillings left, which may be divided equally among the eight persons in coin without any change being required.

142.—­THE SILK PATCHWORK.

[Illustration]

Our illustration will show how to cut the stitches of the patchwork so as to get the square F entire, and four equal pieces, G, H, I, K, that will form a perfect Greek cross.  The reader will know how to assemble these four pieces from Fig. 13 in the article.

[Illustration:  Fig. 1.]