In our epidemiology class, we're having to read an article, then back calculate how many people took part in the study. The article is about 4 groups of women, all with breast cancer:
2 groups that were screened : 1 modern and 1 historical
2 groups that were not screened: 1 modern and 1 historical
It gives the following:
rate of death was reduced by 7.2 deaths per 100,000 person-years in the modern screening group versus historical (rate ratio .72)
rate of death was reduced by 4.8 deaths per 100,000 person-years in modern non-screening group versus historical (rate ratio .82)

The study took place over 9 years for all groups. (historical was from 1986-1995, modern was 1996-2005). It also says that a reduction 2.4 deaths (1/3 of the 7.2) were attributed to screening alone.

How do i calculate from this how many total people took part in the study?