Question 1033112
let speed of B be x km/h
and that of A = y

after 2/3 hour B&A are together 

catchup speed *catchup time =8

catchup speed = x-y

catchup time = 2/3
d=8


2/3(x-y) =8

x-y =12

d=10
let t be the time they are 10 km apart

t(x-y) = 10

12t = 10

t = 10/12 = 5/6 

5/6 +2/3 = 9/6 = 1.5 hours

3:40 +1:30 = 5:10 pm

At 5:10 they are 10 km apart