One decade equals ten years, and nine decades equal 90 years.
Just nine...
There are 3.5 decades in 35 years
There are 5 decades in 50 years.
There are 12 decades in 120 years.
96 decades and 9 years.
There are 19 decades in 190 years.
230 years = 23 decades
There are 20 years in two decades.
1.2 decades
There are 370 years in 37 decades
If you mean decades then there are 10 decades in 100 years
10 years = 1 decade 20 years = 2 decades 30 years = 3 decades . . etc.