an exercise from Spherical Astronomy by Smart I want to solve this exercise from Spherical Astronomy by Smart:

Prove that, in latitude 45°, the interval between the moment at which a star's azimuth is 90° east and the moment of setting is constant.

I can't understand it, it says prove that it is constant for a star, or it is constant for every star with different declinations?

Thanks.
Constant for every star with different declination's.

Though you would have to assume the the declination is such that it can have an azimuth of 90 each, not all stars will. Depending on your latitude some stars will never get to an azimuth of 90, they will either be always be further north or south.