Amber's birthday party will cost $12 if she invites 4 guests. If there are 7 guests, how much will Amber's birthday party cost (c) ? Assume the relationship is directly proportional.

1) We know it costs $12 for inviting 4 quests, so the cost for inviting only one guest would be $12/4 = $3.

2) And it is given that the relation is directly proportional, hence cost of inviting 7 guests would be 7 * cost of inviting one guest = 7*$3 = $21