A 520 m long train running at a uniform speed, crosses a platform in 40 seconds and a person standing on the platform in 26 seconds.  What is the length (in meters) of the platform?

280 m

While crossing the man:

Length of train = Distance = Speed × Time

520 = S × 26

S = 20 m/sec

While crossing the platform:

(520 + P) = 20 × 40

P = 800 - 520

P = 280 m