Two mothers and two daughters went shopping. each bought a dvd with a favorite film and they brought home 3 dvds. how can thet be?

Напишу на русском: 2 матери и 2 дочери(дочь,мать,бабушка)(допустим: Х,У,С...У и дочь и мать,С и дочь и мать)
These peoples are daughter, mum and grandmother