A chemist has one solution that is 60% acid and another that is 20% acid. How much of each solution is needed to make a 200 mL solution that is 50% acid?

Solution:

We assume we need X ml 60% acid and (200-x) ml 20% acid

so 60%*X+ (200-x)*20% = 200*50%

X=150

so

60% acid: 150 ml

20% acid: 50 ml