The given condition is- A bottle contains 20% milk and the rest water. The bottle has 1 liter of water.
Part A:
Let the amount of mixture in the bottle be = x ; mixture will be amount of water plus milk
Given is- the bottle has 1 liter of water and 20% is milk. 20% is 0.2
Hence equation will be:

Part B:
Solving the equation


= 1.25
Hence, total mixture is 1.25 liters.
Water is 1 liter so milk will be =
Or it can also be solved using milk percentage of 0.2%

Hence, milk is 0.25 liter