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
