What is the physical formula of echo ranging? 3Q

What is the physical formula of echo ranging? 3Q

1. A car is driving towards a cliff at the speed of 10 m / s. when the driver passes by a certain place, he hears the echo three seconds later and calculates the distance between the whistle and the cliff? (the sound speed at that time is 340 m / s) 2. A train is driving towards a mountain at the speed of V and honks 700 m away from the mountain. After four seconds, the driver hears the echo and asks the car to drive

The formula of echo ranging
It must be a formula~

Echo ranging measures distance by measuring time
The formula should be l = V × T / 2
Velocity of sound in air V:
The velocity of sound in air increases with the increase of temperature. The velocity of sound in air is 331.4 M / s at 0 ℃ and 340 m / s at 15 ℃. The velocity of sound increases about 0.6 m / s with the increase of temperature