So we can do this by finding how much is needed for one pancake first.

50ml of milk is needed to make 1 pancake.
From there we can multiply it by 6 to figure out how much we need for 6.

Alternatively, you can just divide 400 by 8/6 but some people find that confusing and it's often easier to follow workings if you do the above method.
300ml of milk is needed to make 6 pankakes.