Suppose the cells of a tumor are idealized as spheres each with a radius of 5 m

Suppose the cells of a tumor are idealized as spheres each with a radius of 5 μm (micrometers). The number of cells has a doubling time of 35 days. Approximately how long will it take a single cell to grow into a multi-celled spherical tumor with a volume of 0.5 cm³ (1 cm = 10,000  μm)? Assume that the tumor spheres are tightly packed.