Question 844464
Assuming 12 hour format, the values would form H:M for hours on the left of the colon and minutes on the right side.  The maximum value on the left would be 12.  She sees M=2H.  


If she saw the maximum H=12, then the corresponding M=2*12=24.

H_____________M=2*H
12____________24
11____________22
10____________20
09____________18
08____________16-----------That gives sum of the digits to 15.
----------------------------------8+1+6=15



That is crude, but it works.
The digital clock shows 08:16.