Question 320038
speed of robber = 80mph
speed of police = 90mph
robber has traveled 40 miles when the police start.( police start 1/2 hour later)
Let them meet at distance x from the bank.
time taken by police = x/90 hours
time taken by robber in the same time = x-40/80. 
..
x/90 = x-40/80
80x=90(x-40)
80x = 90x-3600
10x=3600
x= 360 miles. The distance where they catch up with the robber  from the bank.
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speed of police = 90mph
distance = 360 miles
time they catch up = 360/90
= 4 hours