To subtract, we may proceed in either of two ways.
To add or subtract decimals, we must reduce them to the same denomination, i.e.
She can add and subtract with great rapidity up to the sum of one hundred; and she knows the multiplication tables as far as the FIVES.
Similarly we cannot subtract 8 from 15, if 15 means 1 ten + 5 ones; we must either write 15-815-8=(10+5)-8= (I o - 8)+5 = 2+5 = 7, or else resolve the 15 into an inexpressible number of ones, and then subtract 8 of them, leaving 7.
Thus by adding 2 to aN we can subtract N from aN+2, obtaining 2000-0000, which is =2.