The wave crests generated in a ripple tank are 15mm apart and the frequency of the vibrator is 20Hz.

If the frequency of the vibrator is trebled what is the spacing between the crest

2 answers

divide 15mm by 3
Yes
lambda = v/T = f v
If velocity is constant that works. However watch out. In water gravity waves that only works when the depth is shallow compared to the wavelength in which case velocity is proportional to square root of depth.
When the water depth becomes greater than half a wavelength, the velocity becomes proportional to the square root of the wavelength