This can be solved by partitioning one of the numbers. Here are two tens frames showing 8 orange counters. Here are 4 yellow counters which are going to be added to the 8 orange counters.

The Russian abacus (tchoty) is held so that counters move crosswise. It has no crossbar, and all counters in one row have the same value. Most rods have ten counters. An occasional row has fewer, to ...