Question 989003
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P=price/kg of prawns; C=price/kg of crabs
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4kg(P)+3kg(C)=$114 Multiply this by 3.
12kg(P)+9kg(C)=$342 Call this Equation 1. 
3kg(P)+5kg(C)=$135 Multiply this by 4.
12kg(P)+20kg(C)=$540 Call this Equation 2.
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Subtract Equation 1 from Equation 2:
. 12kg(P)+20kg(C)=$540
-(12kg(P)+ 9kg(C)=$342)
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11kg(C)=$198
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C=$18/kg
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ANSWER: One kilogram of crab cost $18.