Physics - Level 2 - 5. Prob 8. - Escalator in a shopping mall - relative motion

This is the second problem related to relative motion and relative speed.
See the introductory video and previous problem (links below).
Problem:
A customer climbs an escalator in a shopping mall at a constant speed and it takes him 10 s. When the escalator is stationary due to a failure, a customer moving at the same speed relative to the stairs as before climbs the escalator in 30 s. Calculate the time, in which the customer would climb this (operating without failure) escalator while standing on them.
The problem is inspired by a similar problem found in the textbook:
[in Polish] W. Mroszczyk, J. Salach, Zbiór zadań z fizyki. Kurs podstawowy z elementami kursu rozszerzonego, koniecznymi do podjęcia studiów technicznych i przyrodniczych. Część 1. ZamKor. Kraków (2005)
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