Half of 1 litre = 500 mL
Quarter of 1 litre = Half of half litre = 250 mL
Three quarters of 1 litre = Half of one litre and a quarter of 1 litre
First bottle can hold three quarters of a litre = Half of one litre and a quarter litre
= 500 mL + 250 mL = 750 mL
Second bottle can hold one quarter of a litre = 250 mL