Question 73967
y = # gal of high octane
x = # gal of low octane

y is worth .63 per gal
x is worth .51 per gal

mixture = 210 gal

y + x = 210
.63y + .51x = V

To write this as a function of V we need to eliminate one of the variables.
From the first equation, we get...

y = 210 - x

Lets substitute that into the second equation.  V is a function of x.

V(x) = .63(210 - x) + .51x
     = .63(210) - .63x +.51x
     = 132.3 - .12x