Selling price of coffee mixture 
A trader buys $15$ Kg of Arabian coffee powder at $x$ dollars per kg and $25$ kg of Brazilian coffee at $y$ dollars per kg. He mixes the two types of coffee powder in a ratio $3 : 5$ and packs the mixture into packets each of which contains $100$ grams of the mixture. He sells the packs for $ 40x + 48 y / 640 $ dollars. Find an expression for the selling price of all the packets of coffee powder in terms of $x$ and $y$.

I clearly knows that I need to find the cost price of the coffee powder . 
I done up a ratio of the price of the coffee powder = $ 3x : 5y $ 
Why is the cost price of the coffee powder not $ 3x + 5y $ ? 
Can I get a hint on how to carry on? Thanks in advance ! 

It cost the dealer 15x+ 25y dollars for the entire 15+ 25= 40 kg of coffee.  That is an average price of (15x+ 25y)/40= 5(3x+ 5y)/40= (3x+ 5y)/8 dollars per kilogram.
