As a decade is 10 years and there are 42 years between the two years given, then the answer is 4 decades. If you are looking at fixed decades, you could say the 80s, the 90s, the 00s, and so say 3 decades. So it depends on what way you want to specify your decades.
Ten decades.
It is 3 decades, because it is 30 years.
7 decades which is 70 years
5!
8
A decade is 10 years, 200 years between 1776 and 1976 so the answer is 20 decades.
100 decades in a millennium
A decade is 10 years. Since there are 20 years from 1942 to 1962, that would be 2 decades.
university-building in the formative decades between 1880 and 1910
There are 20 decades.
16.7 decades
fifty decades