overall 31% 100

C/W 0% Worth 30% overall

Exam ?% Worth 70% overall

Can anyone work out the exam result and then tell me.

You add up products of percentages and their weights:

percentage * weight + percentage * weight + percentage * weight +... = overall percentage

In your case, there are only two categories:

(CW percentage) * (CW weight) + (exam percentage) * (exam weight) = overall percentage

Plug numbers in :

0 * 0.3 + x * 0.7 = .31

Now solve for x.