it says the t-test was carried out on the results for
pH4.0 AND pH5.2....and the cultures of EACH pH were sampled at
hourly intervals of 20hours...so for pH4.0, the number of degrees of freedom is 20-1=19. for pH5.2, the number of degrees of freedom is 20-1=19. therefore, the number of degree of freedom of THE TEST is the sum of the two number of degree of freedom, which is
19+19=38....
that's what I understand from the question...correct me if I'm wrong....