It took jen, who biked at a speed of 12 mph, 20 minutes to get home. what is jim’s speed, if it took him only 15 minutes to bike that distance?

1)  Speed = 12mph =12 mi/1 h =12 mi/ 60 min = 1/5 mi/ min 
1 h= 60 min
t =20 min
Distance=speed*time =1/5 *20 (mi/min *min)= 4 mi

2) If he would bike for 15 min
Distance=speed*time 
speed = distance/time = 4mi/15 min =4 mi :(1/4 h)=4*4= 16 mi/h
15 min =1/4 h