Question 640393
First express the problem in terms of the amount of acid  in the acid solutions.
 For example, in 10ml of a 10% acid solution, there are 10% of 10ml (0.1(10)) of acid which gives us 1ml of acid.
For this problem, if we lex x = the number of ml of 10% acid solution and (100-x) = the amount of 15% acid solution, we can express the amounts of acid thus:
0.1x+0.15(100-x) = 0.12(100) Simplify and solve for x.
0.1x+15-0.15x = 12
-0.05x+15 = 12 Subtract 15
-0.05x = -3 Divide by -0.05
x = 60ml
The chemist will need to mix 60ml of the 10% acid solution with (100-x = 40)ml of the 15% acid solution to obtain 100ml of 12% acid solution.