Essential Genetics 6th Edition by Daniel Hartl

Exercise 4

In familial retinoblastoma, there is an average of three retinal tumors per heterozygous carrier of the mutation. Assuming that the number of retinal cells at risk is
in each eye and that each tumor results from an independent loss of heterozygosity, what is the estimated rate of loss of heterozygosity per cell? Should this be regarded as a "mutation rate"? Why or why not?