F + S = 11 because the total time is 11 hrs
12F = 10S and the distance he travels at each speed is the same
where F is the time at 12 mph and S is the time at 10 mph.
Substitute from one equation into the other ... and one way is no easier than the other.
For example: F = 5S/6
(5S/6) + S = 11
You solve it from here.