Adam has $15$ of a certain type of rare coin and is interested in knowing how much this collection is worth.  He discovers that $5$ of these coins are worth $12$ dollars in total. Assuming that the value of each coin is the same, how many dollars is his entire collection worth?
To solve this problem, I need to find the value of one coin and then multiply it by the number of coins Adam has. I can use the information that $5$ coins are worth $12$ dollars to find the value of one coin. If I divide $12$ dollars by $5$ coins, I get $2.4$ dollars per coin. Now I can multiply $2.4$ dollars by $15$ coins to get the value of the whole collection. I do $2.4 \times 15$ and get $36$ dollars. Therefore, Adam's coin collection is worth $36$ dollars. 