Suppose that a person with seven friends invites a different

Suppose that a person with seven friends invites a different subset of three friends to dinner every night for one week (seven days). How many ways can this be done so that all friends are included at least once?

Solution

Each friend will meet 2 others at one dinner

mostly three dinners for each friend

7 days and therr friend per a day

Each friend will be invited exatly three times