Answer: 0.6
-------------------------------------------------------------------------------
To get this answer, subtract the given value (0.4) from 1 to get
1-0.4 = 0.6
The reason why this works is because the event either happens or it doesn't happen. There are no other options. So if
A = event happens
B = event doesn't happen
then P(A)+P(B) = 1 which leads to P(B) = 1-P(A)
