2 mile is the distance between A and B

Jana walks at a speed of 6 miles/hour

Jana takes the time of  20 min to go from A to B

6/3=2 miles

60/3= 20 minutes

Zhao starts after 5 min but goes at the speed of 10 mile/hour

Zhao takes the time of 

10/5=2 miles

60/5= 12 minutes

But when Zhao starts Jana already finished 5 minutes of her 20 minutes which means she takes 15 minutes to reach point B whereas Zhao takes 12 minutes Therefore

15-12=3

Jana will reach B 3 minutes after Zhao