Let x and y be the required amounts of the 35% and 60% acid solutions, respectively.
x liters of 35% acid solution contains 0.35x L of acid.
y liters of 80% acid solution contains 0.80y L of acid.
Together, a combined (x + y) L of mixed solutions contains (0.35x + 0.80y) L of acid.
You want to end up with 60 L of 65% acid solution, which means
x + y = 60
0.35x + 0.80y = 0.65 × 60 = 39
Solve for x and y :
y = 60 - x
0.35x + 0.80 (60 - x) = 39
0.35x + 48 - 0.80x = 39
0.45x = 9
x = 20
y = 40
