A person standing on an escalator takes time t1 to reach the top of a tower when the escalator is moving.he takes time t2 to reach the top of the tower when the escalator is standing.how long will he take if he walks up a moving escalator?
a)t2-t1    b)t1+t2    c)t1t2/(t1-t2)  d)t1t2/(t1+t2)

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