Help with microbiology homework?

We did an experiment where we did 10^ -7, 10^-8 dilutions. For the first tube of bacteria we grew 38 colonies after pipetting 1mL to plate with 10^-8 dilution. And I grew only 3 colonies after pipetting 0.1mL with a 10^-7 dilution. How do I calculate the INITIAL number of organism in a flask?