40 litres of 60% concentration of acid solution is added to 35 litres of 80% concentration of acid solution. What is the concentration of acid in the new solution?

Acid in 40 litres mixture = 40 $$\times \frac{60}{100}$$ = 24 litres
Water in 40 litres mixture = 40 - 24 = 16 litres
Acid in 35 litres mixture = 35 $$\times \frac{80}{100}$$ = 28 litres
Water in 35 litres mixture = 35 - 28 = 7 litres
Total acid in the new solution = 24 + 28 = 52
Total water in the new solution = 16 + 7 = 23
Concentration of acid in the new solution = $$\frac{52}{52 + 23} \times 100 = \frac{52}{75} \times 100 = 69 \frac{1}{3}$$%