Answer:
2/5 (or 2/5th) of the registered voters did not participate in the 2016 election for the state
Explanation:
The total probability is 1 (if you add the fraction who did participate and the fraction that didn't, then you get 1), and since you have 2 choices, either you participate or you don't participate in the election, we conclude that the remaining fraction is,
(fraction of Those who didn't participate) = 1 - (fraction of those who did participate)
fraction of Those who didn't participate = 1 - 3/5
fraction of Those who didn't participate = 5/5 - 3/5
fraction of Those who didn't participate = 2/5
Hence, 2/5th of the registered voters did not participate in the 2016 election for the state