Answer:
40 14/25 years
Step-by-step explanation:
Well, this certainly is fun.
First, to find the fraction of the doctor's life which 13 years represents, subtract the fractions from 1 (and in this case l stands for lifetime):
[tex]13 = (1 - (\frac{1}{13} + \frac{4}{39} + \frac{1}{2}))(l)[/tex]
[tex]13 = (\frac{25}{78})(l)[/tex]
So, multiply this equation by the reciprocal of 25/78 to isolate one of l:
[tex]13(\frac{78}{25}) = l[/tex]
[tex]l = 40\frac{14}{25}[/tex] years