# Calculating percentages with weights

I've done a search for the answer but still couldn't figure it out.

Say for example:

Year 1 Result = 80% (Weight:20%)

Year 2 Result = 70% (Weight:10%)

Year 3 Result = Unknown (Weight:70%)

I've not yet got my results for year 3. I want to calculate what my average percentage is so far, based on year 1 and 2 and what I would need to achieve in my 3rd year with a weighting of 70%.

Thanks.

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What are you trying to get? I assume you are trying to weigh grades in a class or something and trying to find out what you need on Year 3 to get a certain grade in the class. If you provide what you are trying to get we can help. – Dan W Oct 17 '11 at 18:48
I'm trying to achieve a 70% average across 3 years with those weightings. Thanks – Henryz Oct 17 '11 at 18:54

Imagine there are 100 points available, 20 of them in year 1, 10 in year 2, and 70 in year 3. You got 80% of 20=16 in year 1 and 70% of 10=7 in year 2. So now you have 23 and need 70. You need 47, which is about 67% of 70.

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Since you are trying for $70\%$ total, you equation looks like:

$$0.7 = 0.8*0.2 + 0.7*0.1 + x*0.7$$

Then, when you solve for $x$, you get:

$$x = 0.67$$

Meaning, you need to get a $67\%$ on Year 3 Result to get $70\%$ for all three years.

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