Well it can be written simply as A+B+a+b=1

where A=probability of A

B=prob of B

a=neither

and b=both

the both can be written also as the probability of A x prob of b

ie. b=AB

therefore

A+B+a+AB=1

A(1+B)=1-B-a

A=(1-B-a)/(1+B) which is almost what you had in the question but i think this should be the answer

3. The probability of the dice can be written as follows;

1=x

2=x

3=x

4=2x

5=x

6=4x

therefore

10x=1

x=0.1

and therefore as the even numbers add up to 7x the prob is 0.7

Hope that helps

Chris