additive color color theory color wheel subtractive model wheel add adding-machine addition appearance bulky buttons calculate calculating calculations calculators clear Clearance computers computing digits handle heavy input large limitations limited machine numbers numerals office office-equipment output predating press pressing problem-solving problems slow subtract subtraction technology additive model subtractive arithmetic basic business count digit digital display math minus multiply number plus pocket children division education kindergarden kindy learning mathematics maths multiplication play playing school toy 10 accounting game list point random rate score ten tile 1 2 3 beginning child class clay colour counting elementary grade homework introduction kid kids kindergarten learn lesson modelling numerical order play doh preschool primary schoolwork sequence shape student study subject teach teacher