Answer:
3 Liters of the 85% Solution
Step-by-step explanation:
You have to draw your buckets. I can't on computer so imagine the fraction bar is the bottom of the buckets.

Then we multiply the top of the buckets with the bottom:
85x+330=65x+390
Now we simplify and solve
20x=60
x=3
You need 3 liters of the 85% solution!
So we can make an equation for this, being 7=1/3x, as that effectively says she can make 3 drawings per page.
Knowing this, we can multiply it by 3 to get x=21 (since if she can draw 3 drawings per page, we can imagine that if each drawing took a page she would need 3 times the pieces of paper), therefore she can make 21 drawings.
If 5 biscuits cost 40p
1 biscuit would cost (40p divided by 5) 8p
8p multiplied by 3 (to get 3 biscuits) would cost 24p
3 biscuits cost 24p