Given:
$90 good for 3 pax for 1 week.
a) $90 / 3 = $30 cost to feed each person in a week
b) $30 cost to feed each person * 5 persons = $150
c) $30 cost to feed each person * 6 persons in a family = $180
The formula derived from the above facts is:
Total cost = cost per person * no. of person * no. of weeks.
Assuming = 6 persons for 2 weeks
Total cost = 30 x 6 x 2
Total cost = 180 x 2
Total cost = 360 cost to feed a family of 6 in 2 weeks.