Question 726611
A food store makes a 11–pound mixture of peanuts, almonds, and raisins. The cost of peanuts is $1.50 per pound, almonds cost $3.00 per pound, and raisins cost $1.50 per pound. The mixture calls for twice as many peanuts as almonds. The total cost of the mixture is $21.00. How much of each ingredient did the store use? 
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You need 3 equations:
Weight: p + a + r = 11 lb
Value: 1.5p+3a+1.5r = 21 dollars
Quantity: p = 2a
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Substitute 2a for "p" to get 2 equations with 2 variables.
Solve the system for a and r: then for "p".
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Cheers,
Stan H.