Agroup of researchers are interested in conducting a clinical trial to determine whether a new cholesterol-lowering agent was useful in preventing coronary heart disease (chd). they identified 12,327 potential participants for the trial. at the initial clinical exam, 309 were discovered to have chd. the remaining subjects entered the trial and were divided equally into the treatment and placebo groups. of those in the treatment group, 505 developed chd after 5 years of follow-up while 477 developed chd during the same period in the placebo group. what was the point prevalence of chd at the initial exam