A Chemist has 100g of 25% acid solution. How much of these solution he needs to drain and replace with 70% acid solution to obtain 100g of 60% acid solution?

To solve this kind of problem, there are two concepts: the total amount of solution and the total amount of acid

in this case, suppose there are x grams of the 25% solution, y grams of the 70% of soltuion

x + y = 100 = > y = 100-x

0.25x + 0.7y = 0.6 * 100 = 60

replace y with 100-x: 0.25x + 0.7 (100-x) = 60

0.45x=10

x = 22.22

100-22.22=77.78

so he needs to drain about 77.78 grams of the solution.