Glioblastomas are the most malignant type of brain tumor. Patients diagnosed with glioblastomas live, on average, 14 months after diagnosis. The lethality of glioblastomas is largely due to its ability to develop resistance to chemotherapy. The mechanisms of cell repair responsible for chemotherapy resistance are very complex and not well understood. We present a mathematical model that simulates the treatment of glioblastoma with standard chemotherapy (Temozolomide). Our model explores detailed mechanisms of drug actions and the different repair pathways that a tumor cell uses to resist the effect of chemotherapy. Using a genetic algorithm in a virtual cohort we predict the potential of combining chemotherapy with DNA-repair enzyme inhibition in improving patient survival times.