Question

how much 78 octane gas and 91 octane gas should be blended to make 13 gallons of 85 octane gas

Answer 1

13 x 85 = 1105
78 + 91 = 169

169 / 1105 = 6.5

Answer 2

Simultaneous equations
x+y=13
78x+91y=85(13)
(x,y)=(6,7)