Problem

If 1500 white blood cells are lined up side by side, they would form a row 1.0 inch long. What is the average diameter of a single white blood cell?
(1 inch = 2.54cm)

Set up a proportion.
1500/2.54 = 1/x
x is the length of 1 white blood cell. Seems like a large number to me. 1.7 mm seems too big but then I don't know much about white blood cells.

I missed my estimate by a factor of 10. That's about 0.0017 cm or 0.017 mm or 17 um if I didn't goof.