Adoctor orders 20 grams of a 52% solution of a certain medicine. the pharmacist has only bottles of 40% solution and bottles of 70% solution. how much of each must he use to obtain the 20 grams of the 52% solution?

Try, please, this:
1) Imagine, that pharmacist used 'x' gr. of 40% solution and 'y' of 70% solution.
Together (20 gr. of 52% sol.) it will be x+y=20.
2) From other side, the pharmacist combined 40x and 70y gramms, after obtaining together it is 52(x+y) gramms. In other words 40x+70y=52(x+y)
3) 

8 gr. of 70% solution and 12 gr. of 40%.

The answer is 6 weeks