Answer :
Make a system of two equations, with variable x = number of a gallons from the first source and y the number of gallons from the second one.
(1)x + y = 80 gallons
0.65 x + 0.90y = 0.75*80
(2) 0.65x + 0.90y = 60
From (1) y = 80 - x
Substitute in (2) 0.65x + 0.90(80 - x) = 60
0.65x + 72 - 0.90x = 60
0.25x = 12
x = 48 gallons
y = 80 - 48 = 32 gallons
Answer: 48 gallons of the first source and 32 gallons from the second one.
(1)x + y = 80 gallons
0.65 x + 0.90y = 0.75*80
(2) 0.65x + 0.90y = 60
From (1) y = 80 - x
Substitute in (2) 0.65x + 0.90(80 - x) = 60
0.65x + 72 - 0.90x = 60
0.25x = 12
x = 48 gallons
y = 80 - 48 = 32 gallons
Answer: 48 gallons of the first source and 32 gallons from the second one.