Let the amount that is left in the big one be x AFTER the amount has been tipped into the cup.
Which means that 1/6 of x is tipped into the cup
which means that originally there was 7/6 of x in the jug.
So the cup now has x/6 ml in it If we add 50ml that will be x/6+50 millilitres
and we are told htat that will be 1/5 of what is now in the pitcher (how much was that again)
Draw a pic to help you see what is happening.
Set up a simple equation, solve for x, then solve for the amount of water that was in the pitcher in the first place.