A student used a ruler to measure the field view at x100 as 1.6 mm. They then viewed cells at a magnification of x400. They estimated that 10 cells could line up across the field of view.
The keys here to consider are:
1) At 400x, would the amount viewed be 4 times as much or 1/4 as much as at 100x?
2) If 10 cells are required to fill ONE field, how much of a field does ONE cell cover?
Combining these values with the fact that one full field (i.e. a diameter of viewing) is 1.6mm (and 1 MILLImeter =1000 MICROmeter) should take you home.
Check your answer: whatever you calculate, does it make sense given that a cel is a FRACTION of a field @400x whereas 1mm is about a field at a LOWER magnification?