An extension of the block of numbers 1--100 to the block of numbers 1--1000 is considered. The reason for singling out this block is a thorough elaboration of the procedures of decimalization. Represented as sums of hundreds and one- or two-digit numbers, addition and subtraction of three-digit numbers is reduced to easy and already known cases of performing these operations. Represented as sums of units, tens and hundreds, these two operations are also carried out on such summands. To the latter case, the shorthand forms of performing these operations on digits, are attached. Bound to the cases of one-digit multipliers or divisors, multiplication and division are treated similarly. By analogy, these acquired skills and understanding are expected to be carried over in cases of all other numbers, when operations are chiefly performed on digits. At the end, we summarize main points of our approach to the elaboration of arithmetic, being exposed in all seven parts of this paper that run under the same title.