Question 243219
You need to provide more information.  For example, what is the cost to remove 1% of the pollution?  Then simply multiply by 75 to get the cost to remove 75%.
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Since you say you have a series of percentages, I assume you have costs associated with each of these.
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Plot these values to see if a straight line will fit.
If so, derive the equation of the line and plug in 75% to get the expected cost.
Note that .75 is the slope of the line = m.
y = mx + b,
where y = total cost.
b would be some constant that is present in all cases.
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If there is a single curve, then you have have to create a quadratic equation to solve it.
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But in any case, we need more info to be able to help more.