And then he showed me, proved by elaborate calculations in his pocket-book, that supposing he had contracted, say, two hundred pounds of debt in six months out of the army; and that he contracted no debt at all within a corresponding period in the army--as to which he had quite made up his mind; this step must involve a saving of four hundred pounds in a year, or two thousand pounds in five years, which was a considerable sum. 
And then he showed me, proved by elaborate calculations in his pocket-book, that supposing he had contracted two hundred pounds of debt in six months out of the army; and that he contracted no debt at all during six months in the army - as he intended; this way he would save four hundred pounds a year, which was a considerable sum. 