Maths S1 Statistics AS-Level homework help

Hey guys I'm really stuck on question B so could use a bit of help, i'll write out the question and what I have done:

7) C and D are two events and P(C|D) = 1/3, P(C|D') = 1/5 and P(D) = 1/4. Find:

a) P(C n D)
For this question I basically substituted 1/3 and 1/4 into the conditional rule and got the answer 1/12. I was fine with this question. However, I couldn't do B:

b) find P(C n D')


if somebody could help or guide me that'd be great

Just replace the D with D'

so:

P(C | D') = P(C n D') / P(D')

but what is the p(d') ???

p(d`) = 1 - p(d)