If the probability of a spinner landing on red in a game is 1/5, what is the probability of it not landing on red?

Pretty simple, *The probability of it not landing on red is 4/5*

P(A)  - probablity of a spinner landing on red
P(A') - probablity of it not landing on red. 
From formula:
P(A) + P(A') = 1     Substitute P(A) =1/5 :
1/5 + P(A')=1 
P(A')=1-1/5
P(A') = 4/5  =4/5 * 100% = 80%
So this probablity is 4/5 (or 80%, if you want to have it on a percent)