I'm completely stumped with this question.

Explain what is meant by the term “index of cell number” What would be the index of cell number ( to the nearest whole number) for the epidermal gamma cells of a 7-day gamma irradiated wheat plant which was known to have a coleoptile length of 14 mm and a mean cell length of 318 μm.

The problem is we were never told how to do this and the method for calculating this quantity doesn't seem to be in any of the supplied course literature.

Does anyone know how to do this? Can you please show me?