What country was the first to use currency based on a decimal unit?
Russia is the first country to use a decimal currency unit. The Russian ruble is believed to have become the first decimalized currency when Peter the Great established the ratio 1 ruble = 100 kopecks in 1701. The Japanese were in some sense earlier calculating with the silver momme and its decimal subunits, but the momme was not a coin. It was a unit of weight equivalent to 3.75 g. To determine the proper momme, it was done using the weight of silver. The British pound sterling was the last major currency to be decimalized on February 15, 1971.
Regarding non-decimal currency, it is currency that has sub-units that are a non-decimal fraction of the main unit, i.e. the number of sub-units in a main unit is not a power of 10. Today only two countries in the world use non-decimal currencies. Both are in Africa. The countries are Mauritania and Madagascar.