p = probability it rains tomorrow
q = probability it does not rain tomorrow
Since it either rains or it doesn't rain, this means p+q = 1
Think of 1 as 100% indicating total certainty
Solve for q to get
q = 1-p
by subtracting p from both sides
We're given p = 0.30 which is the probability of it raining, so,
q = 1-p
q = 1-0.30
q = 0.70
Answer: 0.70