You can put this solution on YOUR website! Learn the distinction between "gas" and "gasoline". A reader could easily take your question to mean 6 gallons of something in the gaseous state of matter. The question may, in fact, mean that, but seems unclear.
But, to the solution.
VARIABLES
T = 88% wanted
L = 87% source
H = 93% source
M = 6 gallons, mixture wanted
u = how much of Low percent gas
v = how much of High percent gas
FORM EQUATIONS and
SOLUTION METHOD
Solve the system symbolically for u and v. Substitute given values to evaluate u and v.
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