Question $3(1$ point$)$

In an experiment similar to Perrin's it is observed that there are $78$ particles per unit volume near the bottom of the column, and $17$ particles per unit volume at a height of $8\mathrm{.}50$ mm $.$ Above what height in the column would you observe less than $1$ particle per unit volume $($hint: think about what height will you expect to find $1$ particle$)?$ Answer in mm to $1$ decimal place but do not include units in your answer.