You have 1kg of unroasted green coffee beans at a cost of $7. When you roast this coffee you lose 20% of the original weight to produce only 800g of roasted coffee. How much is 1kg of roasted coffee? Please teach me this maths!

What part of the problem is giving you trouble?

First, how many grams are in one kilogram?

You lost 20% by weight. What weight of green beans multiplied by X %(remainder) = 1Kg? You then can determine the additional cost to make 1Kg of roasted beans.