really basic %'s

Jul 2010
1
0
overall 31% 100
C/W 0% Worth 30% overall
Exam ?% Worth 70% overall

Can anyone work out the exam result and then tell me.
 
Jan 2010
278
138
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.