If we see a variable in the question that is repeated in the answer choices, there's a good chance we can use Plugging In. Let's make r=2. So Mary travels at 2 mph and Paul travels at 3 mph.
At 12:00, Mary passes the station.
At 1:00, Mary is 2 miles down the road and Paul passes the station.
At 2:00, Mary is 4 miles down the road and Paul is 3 miles down the road.
At 3:00, Mary is 6 miles down the road and Paul is 6 miles down the road. Aha, he caught up! At 6 miles.
Check the answer choices.

(A) 2/2 Wrong
(B) 2 Wrong
(C) 2*2 Wrong
(D) (5*2)/2 Wrong
(E) 3*2

Answer choice E


ThatDudeKnowsPluggingIn
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S * T = D

Mary = r * t= rt

Paul => 3r/2* t-1= 3r/2*(t-1)

rt=3r/2*(t-1) => 2rt=3rt-3r => rt=3r => t=3

Paul will catch up Mary at distance r*t = 3r Ans. (E)