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 ! 

"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? "
It is.  What makes you think it isn't?  Of course, that is not what the question asked so would not be the "answer" to the question.  The question asked for the selling price.
