text: final exam tomorrow plz help!? suppose p(a and b) = 2/15 and p(a/b) = 1/4. what is p(a)? you can calculate p(b) by using the formula   p(a|b) = p(a and b) / p(b)  i believe you cannot calculate p(a) - there is not enough input for this (donjuanin's answer would be correct only if a and b are independent - but you didn't mention this in your question)