a container is filled with 80% milk and 20% water. 5 lit is removed and replaced with 5 lit of water, then 15 lit is removed and replaced with water. Now the % of milk in the mixture is 55%. Find the capacity of the container

Let there be 4x milk and x water initially. Now 5 litre of mixture is removed, 4 of it will be milk and 1 will be water.
So the new ratio would be (4x - 4)/(x + 14). Now we can use the same concept again and then equate it to the given ratio of 55:45 i.e 11:9.
Hence we can find the value of x.
Hope that helps.