​Siri, who is driving her​ automobile, is injured when another driver negligently causes an automobile accident. Siri hires a lawyer to represent her on a 35 percent​ contingency-fee basis. If the lawyer wins the case for Siri or settles the case with​ Siri's approval, he will earn 35 percent of whatever is collected from the defendant. If the lawyer does not win or settle the​ lawsuit, he gets paid nothing. The agent can recover this amount from the principal because of the duty to​ _______.