Abstract: An apparatus comprises a first plurality of first numerical sequences, each numeral within each first numerical sequence being identified by a common indicia and each of the plurality of first numerical sequences having a different indicia; a second numerical sequence of numerals; and a selector which is relatively movable with respect to the first and second numerical sequences for selecting one numeral of the second sequence of numerals. The selector has third numerical sequence wherein each numeral of the third sequence has an indicia which corresponds to the common and different indicia of the first numerical sequence. The common and different indicia is color or pattern selective. In the method, the numeral of the first numerical sequence is a mathematical function of the numerals of the second and third numerical sequences.