I warned you several times and still many of you act as if f lambda = c means that increasing f increases c. This is not true, increasing f merely reduces lambda. This is why I urge you to think of the relationship as lambda = cT = c/f.
Another way to misuse this equation is to suppose that if the wave speed changes as you enter a new medium the frequency must as well. It is the wavelength that changes as the speed changes. The number of pulses per second doesn't change when you cross over. Every you pulse you send from one medium reaches the other and the number of pulses you send per second equals the number of pulses per second that reach the other end of the first medium and the beginning of the new medium. [Unless it is moving towards or away from you, a complication we will deal with next week.]
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