Step-by-step explanation: We start with the fraction 1/180, found by putting 20 seconds over one hour. Then, we need to get the amount of miles the speed of sound travels in 20 seconds. We can use the fraction above as a ratio to get it. 720/1 x 1/180 = 4 miles. Then, we'll just do 4 times 5280, which is 21120.
