Ten
There are 12 decades in a 120 year period.
12
0.1
7 decades which is 70 years
77 decades and 1 year 77.1 x 10 = 771
30 years
20
A decade is 10 years. 2012 is one year. There are no decades in a single year. The years 2002 - 2012 represents a decade.
3 decades = 3 x 10 = 30 years
well a decade is 10 year so 2 decades will be 20 years
30 years = 3 decades so 1 year = 1/30 or 0.0333... lots of 3 decades.
One decade is ten years. That means that 511 decades would be 5110 years, or 511x10. One year has 365 days, and every fourth year is a leap year, so 511 decades would be 1866427 days.