Answer: the extraction is 18.2%.
Explanation:
Let's see few definitions:
TDS = Total Dissolved Solids = solids dissolved in the brew
PE = Percent Extraction = percent of solids extracted from the dry coffee grounds
The percent extraction can be calculated by the formula:
PE = TDS × Rm
where
Rm = ratio between the mass of the brew and the mass of dry grounds
Now, you need to consider the following facts:
1) every 1g of coffee absorbs 2g of water
2) the given brew ratio is 1:15, which means that for every 1g of grounds we use 15g of water
Let's call
x = grams of coffee grounds we want to use
Therefore the mass of the brew will be:
m(brew) = water we use - water absorbed by the coffee
= 15·x - 2·x
= 13·x
Therefore,
Rm = m(brew) / x
= 13·x / x
= 13
Hence, the percent of extraction will be:
PE = TDS × Rm
= 1.4 × 13
= 18.2
