Three vessels 'P', 'Q' and 'R' of same capacity are all full. Vessels 'P' and 'Q' contain mixture of milk and water in the ratios 4 : 1 and 7 : 3 respectively while vessel 'R' contains
only pure milk. The contents of these vessels are poured into a larger vessel 'S'. After drawing 30 litres of mixture from vessel 'S', the final quantity of water left in vessel 'S' is 40 litres, then the capacity of each vessel is :