7 sticle de 2 litri inseamna in total 14 litri
4 sticle de 3 litri inseamna in total 12 litri
deci 36 l-(14l+12l)=10l care au ramas de pus in sticle de jumatate de litru asta inseamna ca Nr. sticle de 1/2 l=10:1/2=20 deci 20 sticle de 1/2 l foloseste bunicul