1. The problem states that a man counts his cowries, representing 1 for each 10 counted. 2. This means he is grouping cowries in sets of 10 and counting each set as 1. 3. In modular arithmetic, counting in groups of 10 corresponds to working modulo 10. 4. Modulo 10 means numbers wrap around after reaching 10, so the remainder when dividing by 10 is considered. 5. For example, if he counts 23 cowries, he has 2 full groups of 10 (counted as 2) and 3 leftover. 6. Therefore, the counting system he uses is modulo 10. Final answer: He is counting in modulo 10.