Calculate how much ethanol to add to petrol to get a desired blend

Question

Take the following problem:

Data:

• I have $20$ liters of petrol in a tank:
• Assume that e85 is defined as $85\%$ of ethanol and $15\%$ of petrol;
• Assume that petrol does not contain ethanol.

Problem:
How much e85 we have to add to the $20$ liters of petrol to get a blend of $40\%$ ethanol and $60\%$ petrol?

So, the only values I have to start with is $20$ liters of petrol and a desired percentage, which in this case is $40\%$.

Answer 1

$$20(0.00) + x(0.85) = (20+x)(0.40)$$

Solving for $x$ will tell you how many liters of e$85$ you need to add. To explain the equation in English, from left to right:

"I currently have $20$ liters of 0% ethanol. I need to add some amount $x$ of 85% ethanol. At the end of the day, I'll have $20+x$ liters of liquid total, and I'd like this liquid to be 40% ethanol."