A hiker determines the length of a lake by listening for the echo of her shout reflected by a cliff at the far end of the lake. She hears the echo 2.20 s after shouting. How long (in meters) is the lake?

Answer 1

speed of sound there? Call it v
(maybe around 350 m/s)

goes twice L
2 L = v T = 2.2 v
L = 1.1 v

Answer 2

The string on a violin has a fundamental frequency of 150.0 Hz. The length of the vibrating portion is 36 cm and has a mass of 0.51 g. Under what tension (in newtons) must the string be placed?

Answer 3

I have showed you how to do these.
speed = sqrt (T/u)
u = mass/length
distance for wave to travel in one period = 2L (because half a wave is fundamental)
T = distance/speed
frequency = 1/T