I can only answer part 2 of your question, "If we multiply two positive numbers, is the result positive or negative?". if you multiply 2 positive numbers your answer would be positive.
If the gourmet shopkeeper mix 15 pounds of coffee beans which costs 3.0$ per pound with 10 pounds of coffee beans costing 4.25$ per pound. The mixture of 25 pounds will cost 3.5$ per pound.
It can be found using 3*x + 4.25*(25 -x) = 25*3.50
