Question 451949
Jared ran a distance d=12mi that consists of two parts d1 and d2 -> d=d1+d2.

{{{d1=8*t1}}} , he ran  8mi/h for t1 hours
{{{d2=12*t2}}}, he ran 12mi/h for t2 hours


8*t1+12*t2 = 21


Since he finished his race in 2 hours it means 


t1 + t2 = 2, from here we have t1=2-t2


If we substitute this into first equation we have:


8(2-t2)+12*t2=21, multiply
16-8*t2+12*t2=21, combine
16-4*t2      =21, subtract 16 from both sides
-4*t2        =-5, divide both sides by -4
   t2        = 5/4


So he ran faster for 5/4 hours or 75 minutes.