Question 808307
How much 30% alcohol, 45% alcohol, 60% alcohol and 95% alcohol could be mixed to make 200ml of 50% alcohol. 
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Equations:
Quantity:: t + f + s + n = 200 mL
Alcohol::: 0.3t+0.45s+0.95n = 0.50*200
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You have 4 variables.
You need two more equation statements to solve.
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Cheers,
Stan H.