An airline flight of 5 hours in the middle latitudes at an altitude of 12 km may result in an exposure of _____ µSv.

Directly picked from White and Pharoah: "An airline flight of 5 hours in the middle latitudes at an altitude of 12 km may result in an exposure of about 25 µSv". White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition