Hi there! On wednesday I'm having a really important test at University and in the practice-book I found a puzzle I just can't solve, it goes like this:

In a coffee storea new coffee is being prepared. It contains: 10kg of a $20/kg coffee and some of $12/kg.How many Kilos of $12/kg coffeemust the mix contain so that charging it $18/kg they obtain 10% profit over the cost?

Sorry if it's grammatically wrong.

Thanx !