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)
Answer: 8 gr. of 70% solution and 12 gr. of 40%.
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)
Answer: 8 gr. of 70% solution and 12 gr. of 40%.