A train started at station S and has been traveling r miles per hour for t hours. In terms of r and t , how many miles from station S had the train travelled 3t/4 hours ago ?
(A) rt/12
(B) 3rt/4
(C) rt
(D) 4rt/3
(E) rt/4
Plug in! Say r = 10 and t = 4 —those will be nice, easy numbers to work with. 
becomes simply 3, so the question asks, given that the train has been traveling for 4 hours, how far the train had traveled 3 hours ago. In other words, how far had the train traveled 1 hour after it left?
Well, that’s easy. It’s going 10 miles per hour, so in 1 hour it went 10 miles.
Which answer choice simplifies to 10 when r = 10 and t = 4? Only E.