Walter builds birdhouses. He spends $$5$ on the materials for each birdhouse. He can build one in $30$ minutes. He is semi$-$retired but earns $$8$ per hour at the local hardware store. He can sell a birdhouse for $$20$ each.

Refer to Scenario $13-11.$ The explicit cost for one birdhouse is

Group of answer choices

$$9.$

$$8.$

$$5.$

$$4.$