A dart board has 10 equally sized slices numbered from 1-10. some are grey and some are white. the slices numbered 1,2,3,4,5,6,8,9 and 10 are grey. the slice number 7 is white. a dart is tossed and lands on a slice at random. let X be the event that the dart lands on a grey slice, and let P(X) be the probability of x. let not X be the event that the dart lands on a slice that is not grey, and let P( not X) be the probability of not X.

We have 10 sections, we can assume that the probability of hitting each of one of them is the same.

we have 9 grey slices and 1 white slice, then the probability of landing in a grey slice is equal to the number of grey slices divided by the total number of slices:

P(X) = 9/10 = 0.90

The probability of not landing in a grey slice is equal to the probability of landing in the white slice, this is:

P(not X) = 1/10 = 0.10

Also, when you have only two events, you can calculate this as:

P(not X) = 1 - P(X) = 1 - 0.90 = 0.10