Q17. To obtain a low frequency standing wave you need a string that is long (which makes the wavelength long) and one with a low wave velocity (which means T is small and/or the mass/length is large). By wrapping the strings you raise the mass of the string.
Q22. The frequency is the number of times per second that the medium is disturbed. When a wave changes media, the incoming wave disturbs the new medium the same number of times/second as it disturbed the old medium. Therefor the frequency is the same.
Q4. The frequency will not be changed for the same reasons given earlier. The wavelength will change because the wave velocity changes.
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