A bag contains ten counters of which six are red and four are green. A counter is chosen at random; its colour is noted and it is replaced in the bag. A second is then chosen at random. Find the probability that at-least one is red.
With the help of answer here, I have computed probability of having both green and this is 4/25.
And with the help of some website having similar question, I computed the probability of having at-least one red as
1 - (4/25) = 21/25.
And this is the right answer (checked from answers given at back of book).
I know that we have to subtract 4/25 but, I am unable to understand that why we have subtracted from 1. Can anyone explain me please ?