Linear Programming word problem

[Asmodieus]

Two gasolines, type A (1.13 per gallon) and type B (1.28 per gallon), have octane ratings of 80 and 92 respectively. Determine the blend of minimum cost with an octane rating of at least 90. What is the minimum cost? (Hint: Let x be the fraction of each gallon that is type A and let y be the fraction that is type B)

I dont understand what they want, what would the constraints be? I havent been able to get started on the problem because I dont really understand what they want, help would be appreciated.

1.13x+1.28y= Cost

x > or = to 0
Y > or = to 0

 

[garf]

this problem has this solution. cost = 1.255$
in order to have you must it study. linear programming. direction sees this
http://en.wikipedia.org/wiki/Linear_programming
these are the equations
cost = 1,13x + 1.28y
x + y =< 1
.80x + .92y =< .90
x >= 0
y >= 0
graphic the polynomial and one of its edges this sulocion