Let's fill in the blanks step-by-step based on the problem:
Ari wants to buy his favorite cookies for 15 friends. If the cookies come in boxes with 5 cookies, how many boxes should he buy?
If the variable b represents the number of boxes Ari wants to buy, then 5b represents the total number of cookies.
We can set 5b = 15, since that's the number of friends Ari has.
Divide both sides of the equation by 5 to isolate the variable, and the solution to the problem is 3 boxes of cookies!