Question: You are interested in cloning the period gene that determines circadian rhythms in the fruit fly….

You are interested in cloning the period gene that determines
circadian rhythms in the fruit fly. You generate a number of
chromosome deletion by X-irradiation and perform complementation
tests with a null allele of period. Results are shown below:

Df(46A-48F) mutant

Df(47E-49A) mutant

Df(48B-49B) mutant

Df(46C-48D) wildtype

Df(45D-48E) mutant

Where does the period gene map?

The answer is 48D-48E, however I am at a loss for how to solve
this. This is how my professor solved it, and it makes no sense.
Any help???