To Take One From Nineteen, So That the Remainder Shall Be Twenty
See how it is done: XIX. (nineteen), by taking away the one that stands between the two tens (XX.), twenty will remain.
A similar catch is to write down nine figures, the sum of which is 45, from that number to take away 50, and to let the remainder be fifteen. The numerals should be added together thus: 1+2+3+4+5+6+7+8+9=45, or XLV., from which take away L. (50), and there will be left XV. (15).