Weighted Average Method:

2.78P + 3.28C = 2.99(P+C)

i.e. 0.21P = 0.29C
i.e. P/C = 29/21

C as a % of total Gasoline = [21/(21+29)]*100 = 42%

Answer: Option A
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Weighted average of groups combined = (group A proportion)(group A average) + (group B proportion)(group B average) + (group C proportion)(group C average) + ...

Let x = the percentage of gas sold at Cortoline
This means that 100-x = the percentage of gas sold at Petrocort

As far as PROPORTIONS go, x/100 is the proportion of gas sold at Cortoline
And (100-x)/100 is the proportion of gas sold at Petrocort

Now plug the values into the weighted averages formula.
We get: 2.99 = (x/100)(3.28) + [(100-x)/100][2.78]
Eliminate the fractions by multiplying both sides by 100 to get: 299 = (x)(3.28) + (100-x)[2.78]
Expand to get: 299 = 3.28x + 278 - 2.78x
Simplify: 299 = 0.5x + 278
Subtract 278 from both sides: 21 = 0.5x
Solve: x = 42

Answer: A

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How far is the average from either values ?
so, that is
2.99 - 2.78 = 0.21
2.99 - 3.28 = -0.29

So, for this we can try
x/100 ( 0.21) - y/100 ( 0.29) = 0
now, you can try plugging in values,

here A makes this 0