Well, the main point you should remember is that we only can hear 20hz that leads to T=0.05sec
which means that the distance leads us to velocity x time.
When calculating, you get 330 * 0.05=16.5 .
The number which you got from the previous step has to me multiplied 4 times (4 minutes) and you will get the answer you need - a) 66.0 m.
Do hope it will help you! Regards!
