A sample of 100 healthy men was followed for the development of prostate cancer. After being followed for 5 years, 20 men of the study sample developed prostate cancer. Another 10 men of the sample were followed for 1 year and then were lost to follow up and we do not have information about their prostate cancer status. The remaining men who never developed the disease were followed for 10 years. Calculate the number of person-years of observation accrued by this population.