Can anyone explain me how to do this math word problems

1)Doreen is a chemist. She needs to prepare 36 oz of a 15% hydrochloric acid solution. Find the amount of 18% solution and the amount of 9% solution she should mix to get this solution.

2)Gabe a nucear physicist needs 30 liter of a 60% acid solution. He currently has a 40% solution and a 70% solution. How many liters of each does he need to make the needed 30 liters of 60 % acid solution?

1) Doreen needs X oz of 18% solution and Y oz of 9% solution. We know the following:

(18% of X) + (9% of Y) = (15% of X + Y)
X + Y = 36

So
0.18x + 0.09y = 0.15(36)
0.18x + 0.09y = 5.4
Divide through by 0.09:
2x + y = 60

So
2x + y = 60
and
x + y = 36
The latter tells us that y = -x + 36. Substitute that into the former:
2x + y = 60
2x + (-x + 36) = 60
x + 36 = 60
x = 24

And since x + y = 36, that means y = 12.

2) Same principle. Gabe needs A liters of 40% and B liters of 60%.
0.4A + 0.7B = 0.6(A + B)
A + B = 30

So
0.4A + 0.7B = 0.6(30)
0.4A + 0.7B = 18
Multiply through by 10.
4A + 7B = 180

And A + B = 30. That tells us A = -B + 30. Substitute that into the other equation.

4A + 7B = 180
4(-B + 30) + 7B = 180
-4B + 120 + 7B = 180
3B + 20 = 180
3B = 60
B = 20

And since A + B = 30, that means A = 10.