I think you can do this way:
Volume = B*h where B is the area of the base.
B= 4 * 10 = 40
V= 40 * 15
V= 600 cm^3
Their frequency stays the same and their wavelength decreases
Sara's results probably do have a higher mean, if we take into account how William is way more willing to pay 20 dollars. Honestly, I think you got your chosen answers right! Just my opinion, as someone inexperienced.
Most of the time its juss common sense where you can point the right measurement with just sense i guess
