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See I have a test combined from 3 segments. The first two are 15% each, and the third is 70%. I can't do the second part, so they told me the scores would be 20% and 80% instead.

Now, the scores I got are 15% out of 15, which is easy, but on the third I got 48% out of 70. How do I know how many percent that is from the ultimate 80%? Would love the help. Thanks in advance :)

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If I understand you correctly, you have a score of $100$% on the first segment and $48/70$ of $100$%, or $\frac{48}{70}\cdot 100$%, on the third. The new weighting gives you $20$% of that $100$ plus $80$% of the $\frac{48}{70}\cdot 100$%: $$0.2\cdot 100 + 0.8 \left(\frac{48}{70}\cdot 100 \right) = 20+ \frac{384}{7}\approx 74.857\text{ %}\;.$$

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I think that question is only about third segment so answer should be $\approx 54.857 $ % –  pedja Oct 24 '11 at 10:36
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@pedja: You may be right, but since that’s easy enough to deduce from the full calculation of the final score, I decided to give the full calculation. Perhaps I should have added explicitly that the second term gives the adjusted percentage for the third segment. –  Brian M. Scott Oct 24 '11 at 10:52
    
considering these comments it should be clear enough –  pedja Oct 24 '11 at 11:03
    
Exactly what I needed! Thank you :) –  Chen Asraf Oct 24 '11 at 22:02
    
Exactly what I needed! Thank you! –  Chen Asraf Oct 24 '11 at 22:05
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