How many liters each of a 10% acid solution and a 80% acid solution must be used to produce 70 liters of a 65% acid solution?

Answer 1

10% acid solution: 21 liters
80% acid solution: 49 liters

Answer 2

10x + 80(70-x) = 65*70
x = 15

so,
10% acid solution: 15 liters
80% acid solution: 55 liters