Discussion

Explanation:

Let Bobby's speed be "a" steps per second and escalator's speed be "x" steps per second.

When escalator is moving up, no. of steps 
N = 80a + 80x (Since Bobby is moving upwards, 80x will be added)

When escalator is moving down, no. of steps 
N = 120a - 120x (Since Bobby is moving upwards, 120x will be subtracted)

By equating the number of steps, 80a + 80x = 120a - 120x.

⇒ a = 5x

⇒ N = 80a + 80x = 80a + 16a = 96a

∴ Time taken when the escalator is stationary = 96a/a = 96 seconds.

Hence, 96

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