Abstract: A base member has a plurality of upwardly facing openings for receiving arithmetic value blocks such as addition or subtraction blocks, a first upwardly facing end opening for receiving positive answer blocks and a second upwardly facing opposite end opening for receiving negative answer blocks. A summing rack including transverse gear teeth is slidably mounted in a longitudinal slot in the base member communicating with each of the recesses. One end of the summing rack has an encoded pattern of holes defining the numerical values of possible positive answers; and the other end of the summing rack has a similar series of holes defining the numerical values of possible negative answers. Each of the recesses which receives an arithmetic value block has a sector gear therein which engages the summing rack teeth and moves the rack a proportionate distance when an addition or subtraction block is inserted into the corresponding recess.