Question 510357
Let
Oil = x gallons
Gasoline = y gallons
Total cost of oil = 15x
Total cost of Gasoline = 3y
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Given
Oil Quantity + Gasoline Quantity = Mixture Quantity
x+y=12
x=12-y ...............(1)
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Total Cost of Oil + Total cost of Gasoline = Total Cost of Mixture
15x+3y=12*8
15x+3y=96 ..............(2)
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Put the value of x from (1) to (2)

15(12-y)+3y=96
180-15y+3y=96
-12y=96-180
-12y=-84
-12y/-12=-84/-12
-12y/-12=-84/-12
y=7

Put the value of x in (1)
x=12-y
x=12-7
x=5
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Oil = x = 5 gallons
Gasoline = y = 7 gallons
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Check
5*15 + 7*3 = 12*8
75+21=96
96=96