A drank 100 ml more than half the milk in a container. Later, B drank 100 ml more than half the remaining milk in the container. C went last and drank 100 ml more than half the remaining milk in the container and it became empty. How much did A drink?

Let the volume of the container be V.
Volume of milk consumed by A = V/2 + 100
Volume of milk remaining in the container = V/2 - 100
Volume of milk consumed by B = (V/2 - 100)/2 + 100 = V/4 + 50

Volume of milk remaining in the container = V/2 - 100 - V/4 - 50 = V/4 - 150

So, volume of milk consumed by C = V/8 - 75 + 100 = V/8 + 25
Volume of milk remaining = V/4 - 150 - V/8 - 25 = V/8 - 175 = 0
=> V = 175 * 8 = 1400 ml
So, volume of milk consumed by A = 800 ml