Suppose a study is proposed to compare a renovascular procedure versus medical therapy in lowering the systolic blood pressure of patients with hypertension secondary to renal artery stenosis. on the basis of results of preliminary studies, the investigators estimate that the vascular procedure may lower blood pressure by 20 mm hg, while medical therapy may lower blood pressure by only 10 mm hg. on the basis of results of preliminary studies, the sd for blood pressure lowering is estimated to be 15 mm hg. a significance criterion of 0 .05 and power of 0.80 are chosen. if they want to do a two-sided test of equal population means with α= 0.05 and β= 0.20, what sample size should they use?