Answer:
To solve this problem using unit rates, we can determine the cost per guest by dividing the total cost by the number of guests.
Given that Sam's birthday party costs $8 for 4 guests, we can find the cost per guest by dividing $8 by 4:
Cost per guest = $8 ÷ 4 = $2 per guest
Now, we can use this unit rate to find the cost of Sam's party when there are 13 guests. Simply multiply the number of guests by the cost per guest:
Cost of Sam's party = Cost per guest × Number of guests
Cost of Sam's party = $2 × 13 = $26
Therefore, if Sam invites 13 guests to his birthday party, it will cost $26.