The array can be shown as 5 groups of 3 showing that 5 x 3 = 15. Or it could be shown as 3 groups of 5 showing that 3 x 5 = 15. This demonstrates that multiplication is commutative. Image caption, ...

The 2,300-year-old matrix is the world's oldest decimal multiplication table. From a few fragments out of a collection of 23-century-old bamboo strips, historians have pieced together what they say is ...