The total cost is $175.
The first thing that you need to do is calculate the per day cost per person.
$60 was spent on a three day trip. $60/2 = $20 per day. There were 4 adults on the trip, so $20/4 = $5. The cost of food is $5 per person per day.
On a trip with 7 adults the formula to solve the total cost is:
7 adults x 5 days x $5/day = $175.
