Answer:
see below
Step-by-step explanation:
distance / rate = time
x = walking distance     20 -x = biking distance
x / 4 m/hr  +  (20-x) / 12 m/hr   =  3 hours
x/4 Â + ( 20-x) /12 = 3 Â Â multiply both sides of the equation by 12 Â
3x  + 20 -x  = 36
2x = 16
x = 8    8 miles walking   then  12 miles biking