Quite unclear; "charging it" means charging the mixture or only the $12 coffee?

Is the $20:$12 including the 10%, or will it be sold at $22:$13.20 ?

Anyhow, leaving the "profit out":

x = kg of $20 coffee

[20x + 12(10 - x)] / 10 = 18

Solve to get x = 7.5 kg

Do you know the answer? We can probably figure out the meaning from that.