Mark earns $12.00 per hour as a server in a coffee shop. If a person orders only a coffee, he is supposed to ask them if they would also like a doughnut. For every doughnut Mark sells in that way, he earns an extra 25 cents. What is the $12.00 he earns per hour an example of?Mark earns $12.00 per hour as a server in a coffee shop. If a person orders only a coffee, he is supposed to ask them if they would also like a doughnut. For every doughnut Mark sells in that way, he earns an extra 25 cents. What is the $12.00 he earns per hour an example of?