P (

b.=15/16, p (a and

b.=3/4 find p (a

And or both means we multiply the given probability situations that are mutually exclusive and exhaustive - they cannot occur at the same time.

let P (a) = x

then: P (b) * P (a) = 3/4

15/16 * x = 3/4

15x/16 = 3/4

therefore x = 3/4 * 16/15

x = 4/5

P (a) = 4/5

hope it's clear