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.