The probability that a 21 year old female died in 1997 was [tex]\dfrac{45.5}{100000}=0.000455=0.0455\%[/tex]. This means the chance of a 21 year old female being alive in 1998 (based on 1997 data) would be [tex]1-0.000455=(100-0.0455)\%=99.9545\%[/tex].
