In a cohort study men were classified as having high or low cholesterol levels. Among the group with low cholesterol levels there were 45 heart attacks in 6000 person-years of follow-up and among the group with high cholesterol levels there were 100 heart attacks in 4000 person-years of follow-up. How much heart attacks could theoretically be prevented among men with high cholesterol if nobody had high cholesterol? State the measure and calculate it