Riddle – Dinner

[سلامي مرادي]

Susan has invited her family to Dinner. As her plates are not the best, Susan wants to buy new plates – one for each person at her Thanksgiving Dinner.

There’ll be a grandfather, a grandmother, two fathers, two mothers, four children, three grandchildren, one brother, two sisters, two sons, two daughters, one father-in-law, one mother-in-law and a daughter-in-law.

At least how many plates does Susan need to buy?