A period of 100 years is called a century.
A period of ten years is called a decade. Sometimes a decade is used more specifically, to refer to a ten-year period that starts with a year that ends in zero. For example, "the sixties" or "the 1960s" refers to the years 1960-1969.
ten-year
The term is "decade".
Twenty of anything is called a score. This can be years, eggs, cars, whatever. Although this term is still used and recognized, it is becoming antiquated. If you're looking for a stand alone word for 20 years (i.e. not including the word years) there isn't one, but a decade is 10 years, so you could say two decades. A bit more on score: Score is singular when used for a specific quantity and plural for indeterminate quantities, just like hundred, thousand, million, etc. For example: ten score cars went over the bridge; two hundred cars went over the bridge; scores of cars went over the bridge; hundreds of cars went over the bridge.
300 million years ago corresponds to the Paleozoic era, specifically the Carboniferous period. This period was characterized by the diversification of early reptiles, the formation of large forests, and the development of early amphibians and insects.
About 600 years, from 300 B.C.E. to about 300 C.E.
A Period of ten years is called a decade.
A period of 75 years is called a "septuagenary."
A period of 10 years is called a decade.
A period of 100 years is called a century.
A period of 100 years is called a century.
A period of ten years is a decade.
The period of 100 years is called a century. A period of 10 years is a decade, and 1000 years is a millennium.
A period of forty years is called a quadragennial.
75 years is equivalent to 3/4 of a century
A period of seven years is called a septennium.