Aflu outbreak occurred in a military barracks that housed 20 soldiers. case a began on october 1 and case b was diagnosed on october 2. after approximately 10 days, 12 additional cases occurred during approximately a one-week time span. military epidemiologists believed that this second group cases represented the same generations of cases, and was in the second incubation period after the occurrence of cases a and b; none of the 20 soldiers was known to be immune. calculate the secondary attack rate using the foregoing information.