A 22-year-old student has to have a physical examination to certify that he is able to participate in interscholastic athletics.During the cardiovascular examination,you report his heart rate as 60 beats/minute and his arterial blood pressure as 118/68 mm Hg.According to echocardiographic study,his end-diastolic volume is 150 mL and his end-systolic volume is 60 mL. From the data,you estimate his pulse pressure to be: