How much orange juice at 42¢ a liter should be mixed with grapefruit juice worth 32¢ a liter to make 200 liters of a mixture worth 36¢ a liter?

if x represents the amount of orange juice used, then which of the following expressions represents the total cost of the grapefruit juice used?

X - volume of .42 cents liter
y - volume of .32 cents liter

we need 200 liters of .36 cents liter

A) x + y = 200
B) .42x + .32y = 200 * .36
B) .42x + .32y = 72 multiplying A) by -.42x
A) -.42x -.42y = -84 Then we add A and B
-.10y = -12
y = 120 liters of .32
x = 80 liters of .42
 
Source:
http://www.1728.org/mixture.htm

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