This question can be solved by TESTing VALUES.
We're told that a train has a length of L and passes a pole in T seconds.
L = 3 (feet)
T = 2 (seconds)
Since the pole is NOT given a length, we have to assume that that value is negligible (and does NOT factor into the calculations).
So, the entire length of the train passes the pole in 2 seconds. Thus, it's traveling 3 feet every 2 seconds.
Next, we're told that this train passes a platform in 3T seconds. This means that the train travels 3(2) = 6 seconds and travels 3(3) = 9 feet.
Those 9 feet are made up of the train (3 feet) and the platform (9 - 3 = 6 feet).
The question asks us for the length of the PLATFORM. Using these VALUES, we know that the PLATFORM = 6 when L = 3.
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