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1. A train speeds past a pole in 20 seconds and speeds past a platform 100 meters in length in 30 seconds. What is the length of the train?

A. 100 meters

B. 150 meters

C. 180 meters

D. 200 meters

Solution:

Let the length of the train be x meters and its speed be y m/sec.
Then,

\frac{x}{y}

= 20
⇒ y =

\frac{x}{20}

∴

\frac{x + 100}{30}

\frac{x}{20}

⇒ 30x = 20x + 2000
⇒ 10x = 2000
⇒ x = 200 meters

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