Question 887104
After 30 minutes, A = 5, B = 0
After 1 hour, A = 10, B = 7.5 (half of 15kmh since he left 30 mins later)
After 90 minutes, A = 15, B = 15
B catches up with A 90 minutes after A leaves (60 minutes after B leaves) at the distance of 15km.