Question

Celine has a bottle that contains 20% milk and the rest water. The bottle has 1 liter of water.

Answer 1

We know the amount of water is 1 liter and the milk is 20% of that. Let y be the total amount of liquid in the bottle.
so
y=1+(0.2*1)

Answer 2

Part A: x - total number of liters (milk+water)
0.2x+1=x
Part B:
0.2x+1=x
0.8 x = 1
X=1 : 0.8=1.25 lit.
Milk:
1.25 x 0.2 =0.25