We know that probability of an event ranges between 0 and 1.
Now by the rule
p(X<a)=1-p(X`>=a` ). Where X is a random variable
So p(X<6)=1-p(X`>=` 6).
e.g. In throwing of dice experiment
Here we see
So, 1-P(X`>=` 6)=1-1/6=5/6.