Back calculate total participants from rate ratios

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?