Question 1174541
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Your "total cost" equation is

    40*C + 5*(10-C) = 26*10


where "C" is the mass of the dark chocolate, in pounds.


From the equation


    C = {{{(260 - 50)/(40-5)}}} = 6.


<U>ANSWER</U>.  6 pounds of the dark chocolate and  10-6 = 4 pounds of mustard.
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Solved.