Today it is the Hindu-Arabic numeral system but in the Middle Ages it was the Roman numeral system.
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Most people do not use it in their daily lives.
You and I use it every day, and have done all our lives.
Melvil Dewey created the Dewey Decimal system. It is a classification system used by libraries, and not what is meant in the question. The decimal number system is the number system that we all use on a daily basis; it was invented by a succession of Arab and Hindu mathematicians beginning about 1400 years ago.
It is the number system we use everyday.
If you mean pie: it is eaten. If you mean the number pi: It is used in all sorts of situations. Whether you use it in "real life situation" mainly depends whether you work in engineering. Most people won't use the number pi in their daily lives.